Presidential Election results - Who can enforce them?

[Munch]

If Trump loses, what does it mean "He won't go quietly."?

This quote is the latest of many in the same vein. But as far as I know, Trump can’t just not leave. If he loses the election, come January 20th, a new president gets sworn in whether he likes it or not. He may (or may not) get his more fanatical followers to do some dumb shit but what is all this scared talk of a constitutional crisis? Please explain it to me as if my advancing age has dulled my facilities because I swear I’m dumber than I used to be.