Good questions all. The definition given is "an unelected delegate who is free to support any candidate for the presidential nomination at the party's national convention." In other words, a person who can swing the vote from the candidate preferred by the public to the one preferred by the party establishment.
Both parties have superdelegates (although the number of Republican superdelegates is relatively tiny), which are really a reflection of a concept found in the US Constitution called the electoral college.
When the Constitution was drafted, the concept of the ordinary citizen having enough political acumen to understand the needs internal and external of the national and state governments was unknown. The US Constitution was in effect a treaty between 13 sovereign nations, the United States. Each of those individual nations was granted a method to elect representatives of their nation as electors of the President. A good parallel would be that of ambassadors to the United Nations voting for the Secretary General of the UN. The amount of burden that might be borne through taxation or other expense was dependent on that state's population and therefore their elector number was geared that way.
After the War Between the States, the Federal government asserted its power over all the formerly sovereign states and a single Union was forged. At that time, the electoral college should have been abandoned since the idea on which it had been designed had ceased to exist, however the power base it granted to the Northern states made it in the best interest of the parties in power to retain it.
What it really amounts to is that superdelegates can tilt the selection of a party nominee one way or another, regardless of the public vote. This is how Hillary Clinton received more delegates than Bernie Sanders even though Sanders beat Clinton by 22% in the popular vote.
Even in the Constitution, the electoral college was a bad idea. In today's society, it is downright wrong. There is no reason the public cannot directly elect their representatives considering all the technology we have. It might not be the wisest thing to do, though. People have a tendency to look at the short term rather than the long term effects of their decisions.
Abdicating personal responsibility to "elected" officials is a bad personal decision. We must take charge of our own lives and prosperity. Stop depending on the government for your well-being and make a life for yourself. The parties aren't interested in anything but elections and how to stay in power. Use your voice in town halls, telephone calls, and one-to-one meetings with the politicians.
They hate confrontation. It might mean losing votes.