Can someone please explain to me what happens if neither Democrat Nominee reaches the 2025 delegates?

If neither candidate reaches the needed 2025 delegates, than the 800 Superdelegates, who are usually prominent U.S. political figures such as state governors, are the deciding factor of the nomination. The Superdelegates can vote anyway they want to and help to define the nominee. Some Superdelegates pre-cast or pledge their vote to a particular candidate, but they may change their vote up until the final Superdelegate vote.