Synchronous sound is when the sound/music matches with actions performed on screen- often when a character plays an instrument the sound will line up with what they are doing.

Synchronous sound can be used for Needle drop moments, with a character listening to music in the story and allowing the audience to listen to it alongside them.

Asynchronous sound is when there is a sound with no visible source on screen. This is often used to create tension as the audience question the source of the sound.