Superdelgates consist of about 700 people, that were not elected, but appointed to be Superdelegates. Superdelegates vote outside of caucuses and primaries. They are free to vote any way they want, and they vote at the convention. Superdegates are selected members of the DNC and Congress. Most of these people do not support Bernie Sanders.
Bernie Sanders and Hillary Clinton have practically tied in everything so far:
Except for Superdelegates:
Superdelegates have named Hillary the front runner, and Republican candidates are already talking about how they will pair up against her.
"Although the Democratic Party wants to give every opportunity to grassroots activists... to participate...superdelegates exist...to make sure that party leaders and elected officials don't have to be in a position where they are running against grassroots activists." - Debbie Wasserman-Schultz, DNC Chair
"I think because of the superdelegates who cast their vote based on electability have serious doubts whether Bernie Sanders could be electable once the GOP starts campaigning against him and putting ads against him. I think Hillary's going to do very well with superdelegates. That means Bernie would have to win a significant majority of the elected delegates." - Ed Rendell, former DNC chair.
The DNC does not want a revolutionary to win. They want a member of the establishment. That's why Bernie supporters will have to come out in numbers.