It's diagetic sound, usually music, when it's coming from something inside the movie. Usually, the characters can hear it, too. It's the opposite of background music.
When Jack Johnson said "the diagetic world is so unclear" he was mocking people who "put the BACKGROUND music behind the news tonight." The news audience needs non-diagetic sound (music) to tell them how to react to news, as if it were a movie. Even movies sometimes have only diagetic sound.
by MarDiablito July 09, 2021