In film, what is diegetic sound?

Diegetic sound is noises that can be "heard" by the character within the film or TV show. So someone talking or a door opening or closing. Non-diegetic sound is the sounds that help create emotions for the viewer. So the creepy music that lets the watcher know that something bad might happen or the triumphant music of a character winning in the end.

https://www.adobe.com/creativecloud/video/hub/ideas/diegetic-vs-non-diegetic-sound.html

Non-diegetic sound consists of all audio elements that do not originate from the film's world and are not heard by the characters. These include not only background music but also voice-over narrations and any sound effect added for dramatic emphasis.