If you’re a casual follower of political news, you may have been a little surprised Thursday when news headlines in the middle of a weekday morning suddenly started blaring ”Donald Trump Clinches GOP Nomination.”

Pundits and party elders have been calling Trump the presumptive nominee for weeks. So what changed? And why does it matter? Here’s a quick explainer on the “magic number” and what happened on Thursday.

The headlines all said Trump got to the “magic number.” What does that mean? To be the nominee of the Republican Party, a candidate needs a majority of the delegates who show up at the Republican National Convention to vote for them. There are a total of 2,472 GOP delegates, so a majority is half of that number plus one, or 1,237 delegates. That’s the number that Trump surpassed Thursday, causing all the hoopla. There weren’t any primaries yesterday or today, so where did the extra delegates come from? Each state has different rules for delegates. Most delegates are bound to vote for a specific candidate on the first ballot at the convention, but some are “unbound.” (They’re kind of like a Republican version of superdelegates.) These folks can vote for whomever they want, but they can also pledge their support for a particular candidate before the convention. Enough unbound delegates from North Dakota, Colorado and Pennsylvania pledged their support to Trump to get him over the 1,237 line. As of Thursday afternoon, our NBC News count now has Trump at 1,238 delegates. RELATED: Trump’s weaknesses overshadow Clinton’s

But wasn’t Trump already the nominee?

On May 4, NBC News declared Trump the “presumptive” nominee of the Republican Party. That’s because after his decisive win in Indiana’s May 3 GOP primary, his remaining competitors - John Kasich and Ted Cruz - dropped out of the race. As the only man left standing, he became the race’s presumptive nominee.

Okay, so is he now the “official” nominee?

Nope, Trump remains the presumptive nominee of the party until he is formally nominated at the Republican National Convention in July in Cleveland. The main thing that’s changed is that he’s assured enough delegates to win on the first ballot, a feat he was on a glide path to accomplish anyway, but one he achieved before the end of the primary season.

Lots of people were saying earlier this year that there was going to be a contested convention. Whatever happened to that? Earlier in the campaign, it certainly looked like Cruz and Kasich might be able to get just enough delegates to keep Donald Trump from getting to 1,237. If his rival candidates had been able to limit Trump’s victories, we certainly could have ended up heading to a clash on the convention floor, with no candidate having a majority of delegates. But after the Indiana contest, it became pretty clear that Trump wasn’t able to be stopped, and both Cruz and Kasich ran out of resources and justification to stay in the race, particularly since both of them were already mathematically eliminated from getting to 1,237 themselves. Even without the unbound delegates who backed Trump today, the real estate mogul would have eventually crossed the 1,237 threshold by winning the remaining primaries in delegate-rich states like California and New Jersey on June 7. Does this mean that the Republican Party is coalescing around Trump? Polling data nerds would tell you that the party has already begun the process of unifying. In April, our NBC News/ Wall Street Journal poll showed that 72 percent of Republicans said they’d support Trump over Hillary Clinton, compared to 13 percent who said they wouldn’t. But in the weeks after Trump secured “presumptive nominee” status, that jumped to 86 percent of Republicans supporting Trump. But there are still people like Paul Ryan who haven’t endorsed Trump. Will this number change their mind? Donald Trump’s campaign hopes so! And they have a little bit of a compelling case, since the campaign was able to convince delegates who weren’t required to vote for Trump to commit to supporting him before the end of the primary season. A senior advisor told NBC News’ Hallie Jackson that today’s news shows “the grassroots is coming together more quickly than some of the leadership might be.” That’s a pretty clear hint to people like Ryan and other Republicans that they should fall in line. But, if Paul Ryan’s statements about his lack of a “timeline” for backing Trump are to be believed, the House Speaker is committed to going at his own pace, not letting Trump dictate the terms of an endorsement.

This article first appeared on NBCNews.com.