Fortnite has taken over.

Videos of people playing the game have more than two billion views on YouTube.

Drake has played it with Ninja, a celebrity among Fortnite fans. The gaming expo E3 hosted a 100-player tournament that included the rapper Vince Staples, the actor Jon Heder and the mixed martial arts fighter Demetrious Johnson.

But what, exactly, is Fortnite? If you are older than, say, 30, and not a teacher or a parent, you might have no idea. This is a guide for the perplexed.

All right, lay it on me.

Fortnite Battle Royale is a video game that allows as many as 100 people to meet on a virtual island and battle it out to be No. 1.