Superdelegates are elected politicians and party leaders. They have a say in who becomes the Democratic presidential nominee. Clinton has a lock on the vast majority of these people (she has 394, Sanders has just 44).
Here is the head of the Democratic party (who happened to be co-chair of Clinton's 2008 run) explaining how this is fair.
And this handy chart from 538 shows what a candidate needs in order to win the nomination.