> The No Fly List is a small subset of the U.S. government Terrorist Screening Database (also known as the terrorist watchlist) that contains the identity information of known or suspected terrorists.

The TSA comment above is pretty straight forward but to be fair the wiki article is filled with instances of speculation.

The way people end up on this list is super opaque, but it's certainly not used to catch wanted criminals.

In fact, the US doesn't have super reliable systems to stop fugitives at airports except on entry.