What is a superdelegate and what is their significance to the presidential election?

A superdelegate is a person who is automatically seated at the Democratic National Convention and who is not pledged to either candidate in advance. In other words, the superdelegates can choose who to vote for. All Democratic governors who are currently in office, as well as Democratic senators and representatives and some party officials, serve as superdelegates. Superdelegates are different than pledged delegates, who are selected using the results of the party primaries and caucuses and who pledge their support to certain candidates in advance. The Republican National Convention also has delegates with automatic seats, but they are not called superdelegates and are restricted to three per state.

The significance of superdelegates to the presidential election is that they can swing the convention to choose a candidate who did not win the popular vote during the party's primary elections. The significance to the election of 2016 is that some superdelegates may choose to vote for Bernie Sanders, even if Hillary Clinton wins the majority of the popular vote. This scenario is unlikely, however, as  Clinton seems to have the superdelegates lined up in her column.