The magic number.
This is how many delegates are required in order to secure a majority and win the Democratic party's nomination for the presidency. Two thousand, three hundred, and eighty three.
Hillary Clinton recently reached that amount of delegates.
Now, for clarification, Hillary only has 2383 if you include superdelegates. Basically, superdelegates are important party leaders who also have a vote in who becomes the party's nominee, and they can vote for whoever they want, instead of being pledged to vote along the lines of the popular vote. The majority of delegates aren't superdelegates, but instead are "pledged delegates," and are assigned by each state after their primary, proportionally based on the way the people in that state voted. Secretary Clinton doesn't have 2383 pledged delegates - she has [LOOK UP THE NUMBER] - but if you count superdelegates, she reaches that number. With superdelegates, Hillary has clinched the nomination.
Bernie Sanders, and many of his supporters, are unwilling to accept this. And although I understand why, it's problematic.
Since superdelegates get to choose who to vote for, many people consider them undemocratic. Personally, I agree. (So does the Democratic Party of Wisconsin, of which I am a member.) The winner of the presidential nomination should be decided by popular vote, not by important party leaders. That said, in the years since primaries started to matter and we had a difference between pledged delegates and superdelegates, the superdelegates haven't ever actually changed who the nominee was from the popular vote. That would be undemocratic, and they know it