When the Apple event that unveiled the iPhone 11 took place earlier this month, one keyword—powered by more than 200,000 tweets—trended ahead of the tech showcase on Twitter. “This is a first. #AppleEvent is happening now yet #MOMO is trending worldwide,” one businessman tweeted. “What is MOMO?!?”

Fellow Twitter users were quick to point out that Momo is not a what, but a who: a member of K-pop powerhouse Twice. The reason for her name trending? Twice had just released the artist’s individual trailer for the new mini album Feel Special, out Sept. 23, and the online K-pop community posted a flood of comments about Momo’s new hairstyle in the 20-second clip.

The occasion offered a snapshot of the anticipation that surrounds the girl group’s musical releases—a regular occurrence given that Twice is one of the most prolific K-pop acts. With more than ten albums and EPs dropped since their 2015 debut, the group has consistently released new music, in Korean and Japanese, at least three times a year. That is an ambitious schedule even for K-pop, where the norm is to promote one or two comebacks annually.

The frequent releases have propelled Twice’s rise to the upper echelon of Korean acts. On YouTube, BTS, Blackpink and soloist Psy are the only K-pop acts who have music videos with more views than those of Twice. Besides regularly topping charts and winning music awards in South Korea, the nine-member ensemble has seen almost unparalleled success in Japan. Earlier this year, Twice became the first K-pop girl group to hold a dome tour in Japan, attracting an estimated 210,000 attendees across five concerts. Given that all performance stops in this kind of tour are held at massive dome venues—the ones in Japan have capacities of around 50,000 each—they offer unassailable evidence of a group’s ability to drive ticket sales.

While Twice has always had a significant international following, 2019 has seen the group make major strides toward establishing itself as a global K-pop leader. Twice embarked on their first U.S. tour in July, marking a new chapter in connecting with audiences outside of Asia. The group’s label, JYP Entertainment, also recently announced a collaboration with YouTube that will produce a docuseries about Twice, creating further opportunity to engage with international viewers.

Most recently, Twice has been recognized by the K-pop community for advancing the conversation about mental health—a subject that remains taboo in much of Asia, where depression is often considered to be a sign of weakness, and mental health services in countries like South Korea to China can be lacking. So heads turned when, earlier this summer, JYP Entertainment announced that member Mina would not participate in the group’s world tour due to “sudden extreme anxiety and insecurity toward performing on stage.” In an industry where stars’ health-related absences from activities are often explained ambiguously, the statement’s openness—later supplemented with a note explaining Mina’s diagnosis of anxiety disorder—was a huge step forward in raising awareness around mental health.

Here’s what to know about the K-pop powerhouse Twice.

How was Twice formed?

Like plenty of today’s buzziest K-pop acts, including Monsta X and IZ*ONE, Twice originated from a reality competition show. The 2015 series Sixteen involved that many JYP Entertainment “trainees”—the term used for aspiring idols preparing for a potential debut—fighting for a place in the group’s final lineup. After a series of challenges testing the contestants’ singing, rapping and dancing abilities as well as charisma, seven individuals were eliminated, with the remaining nine forming a new ensemble.

Why are they called Twice?

When the founder of JYP Entertainment, Park Jinyoung, announced the label’s new girl group, he explained that Twice’s name refers to the act making an impact “once through the ears and once through the eyes.” Quite fittingly, the group’s rise to fame has been propelled by consecutive hits that have a vibrant sound delivered through bright, multicolored music videos.

Twice also has a name for its most dedicated fans—a feature shared by all K-pop acts (BTS’ fans are called “ARMYs,” Blackpink’s are “Blinks”). A follower of the girl group is a “Once.” The members explained the meaning on social media in 2015: “If you love us even once, we will repay your love with twice of our love.”

What makes Twice different from other groups?

Twice sets itself apart from other K-pop groups with cheerful tunes and a colorful style—in the members’ fashion, hair and makeup, and a group concept in which each member is represented by a unique color. Bold choruses and dance moves are distinct features of the title tracks in Twice’s albums. The group’s title tracks almost always go viral, in part because they take inspiration from the latest online trends. The video for the 2016 single “TT”, for instance, imitates the crying emoticon through the signature choreography of fingers shaped like “TT” streaming down the face like tears. Meanwhile, the 2017 single “Likey” alludes to the Instagram culture of posing for the perfect photo to attract the heart-shaped “Like.”

Although Twice has established itself as a top K-pop group by embracing a youthful image, the act’s most recent releases have taken a darker, edgier turn. In their latest music videos “Fancy” and “Breakthrough,” the members swapped bright cheerleading uniforms for black femme fatale-esque outfits and performed more sensual choreography. Whether the group will completely transform from a cute image to a fiercer, more grown-up look is a heavily discussed topic among fans.

Who are the members of Twice?

Jihyo (Full Name: Park Jihyo), 22

The leader of the group, Jihyo became a K-pop idol trainee in elementary school and trained for 10 years before debuting in Twice. She is the group’s main vocalist, with key lines in the choruses of Twice’s tracks. In addition to a strong voice that rarely wavers in live performances, Jihyo is known for her versatile dance skills and overall charisma.

Nayeon (Full Name: Im Nayeon), 23

Nayeon is the oldest member of Twice, and often has the position of “center”—which, quite literally, means the member at the center of performances. One of the strongest vocalists in the group, Nayeon frequently starts Twice’s songs and delivers the chorus measures.

Jeongyeon (Full Name: Yoo Jeongyeon), 22

Jeongyeon trained for five years before launching her idol career as a vocalist in Twice. She is referred to as the group’s embodiment of the “girl crush.” On the show Sixteen, Jeongyeon had breakthrough moments during the photoshoot challenges—fitting as she had initially envisioned a career in modeling.

Momo (Full Name: Hirai Momo), 22

Hailing from Japan, Momo is not only viewed as the best dancer in Twice but one of the best dancers across all female K-pop acts. She was eliminated from Twice’s lineup in the reality competition show Sixteen. But the head of JYP Entertainment, Park Jinyoung, added Momo back to the group before the show ended for her unrivaled performance abilities. The individual recording of Momo dancing to title track “Fancy” released earlier this year has earned nearly 10 million views.

Sana (Full Name: Minatozaki Sana), 22

Sana, another member from Japan, is a vocalist with an infectiously bright personality. In the first challenge of Sixteen, in which contestants were asked to demonstrate their “star quality,” Sana chose not to show singing, rapping or dancing skills. Instead, she delivered a live cooking segment hoping to captivate the audience with her quirky charm—which she regularly shows in performances and variety shows four years later.

Mina (Full Name: Myoui Mina), 22

The only member of Twice to be born in the U.S., Mina moved to Japan as a child. Besides being a singer, she is one of the group’s main dancers. She trained in ballet for more than a decade, which may explain her elegant and serene poise, evident whether she is performing in a music video or speaking at an interview.

Dahyun (Full Name: Kim Dahyun), 21

Even before becoming a member of Twice, Dahyun rose to online fame when a video of her performing what would come to be known as the “eagle dance”—executed with rapidly flapping arms that imitate the bird’s wings—went viral. The dance is said to be the inspiration behind a similar animation in the video game Fortnite. One of the main mood-makers of the group, Dahyun is an expressive rapper with punchy lines.

Chaeyoung (Full Name: Son Chaeyoung), 20

Since Twice’s debut, Chaeyoung has taken the role of the group’s main rapper, adding a fierce and playful energy to her verses. She is also an avid artist who has drawn the cover art for one of Twice’s limited edition albums, as well as sketches of her fellow members.

Tzuyu (Full Name: Chou Tzuyu), 20

Born in Taiwan, Tzuyu is Twice’s “maknae”—a term used for the youngest member in K-pop groups—and was only 16 at the time of the group’s debut. Like Momo, Tzuyu was not initially a part of the final lineup for Twice on Sixteen but was added in the show’s last episode based on viewer feedback through online polls. While she first stood out for her striking appearance—large eyes and a small face are favored according to Korean beauty standards—Tzuyu has grown into a poised performer. The singer has been noted for improvements in her vocal abilities, reflected in an increasing number of lines in Twice’s songs.

How did Twice rise in popularity?

JYP Entertainment has long been a major player in South Korea’s music industry and is partially responsible for introducing K-pop to a global audience. In 2009, the company’s trailblazing girl group Wonder Girls became the first Korean act to appear on the Billboard Hot 100. So anticipation was high when JYP announced the upcoming formation of Twice in 2015. Their debut music video, “Like OOH-AHH,” was the first K-pop debut track to reach 100 million views on YouTube. Since then, nine of the group’s music videos have gained at least 200 million views on the platform.

Outside of their performances, Twice made headlines in 2016 when Taiwanese member Tzuyu sparked controversy for waving the flag of Taiwan on a Korean television show. Chinese Internet users angered by the suggestion of an independent Taiwan called for Twice to be banned from performing in China. The then-16-year-old idol apologized in a widely circulated video, saying “there is only one China,” which further incited outrage among those who value Taiwan’s autonomy. The incident occurred around the time of Taiwan’s 2016 presidential elections, and scholars said it may have added one to two percentage points in the victory of current leader Tsai Ing-wen of the independence-leaning Democratic Progressive Party. The backlash against Tzuyu is one of several instances in which celebrities have faced pressure to espouse public support for the Chinese government or else face potential consequences to their careers.

Where should I start with their music?

To get an understanding of Twice’s fast track to fame, watch the music videos for 2016 viral hits “Cheer Up” and “TT.” The first shows the nine members’ upbeat cheerleading choreography in a school-themed set, while the latter features Halloween-inspired scenes with the artists dressed up as characters from Pinocchio to Tinker Bell. “TT” is also the group’s most-watched YouTube video to date, with nearly 500 million views.

If bubblegum pop anthems are not your cup of tea, play the group’s title track released in 2019, “Fancy.” The electronic beats paired with more mellow melodies display a maturing sound, and the music video captures a sultriness that is rarely presented in Twice’s previous releases.

Write to Kat Moon at kat.moon@time.com.