“The superdelegates are a quadrennial pain, an institutional establishment firewall erected (some say) to keep democracy from breaking out entirely in the Democratic nominating process.”
…which is about as fine a short-hand as any.
As near as I can tell, Superdelegate status is the usual bribe honorarium that the Democratic Party gives to The Politburo officials of The Party, and elected officials of each state. (This is IMPORTANT, as we will see in a moment.)
Superdelegates are supposed to be unpledged (Fair Warning: PDF, and it is from 2008, so rules maybe have changed, but probably not a lot), so it remains a mystery to how Clinton can have so many pledged to her.
And just so you know that Superdelegates are always unswayed by campaign contributions, we present this little bit of journalism from The Hill:
“…Sanders has more votes so far and funding to take him to the convention, but Clinton has 359 superdelegates to his eight. The former secretary of State also is raising record sums for state parties through a Hillary Victory Fund. The money, in a two-person race Clinton expects to win, will help her general election campaign and bolster Democrats down-ballot as well. What that money does now is make her more popular with state parties, like the one in Iowa where a recount of the caucus results was initially refused by the state party chairwoman, who has reportedly driven around with a license plate that reads “HRC2016.”
So you see, campaign donations do not have any hint of impropriety about them. No pay-to-play here.
The Republicans have Superdelegates, too/also, but not as many in number, and have a more top-down control of them, as is the nature of fascists.
UPDATE 1: Vox has an explainer up on Superdelegates.