Literally "robbed time" in Italian, rubato lets a phrase stretch and then catch up. It is a hallmark of Romantic piano music, especially Chopin, and of expressive solo performance in general.
Rubato makes music feel human. It also makes it harder to edit to a strict grid, so it suits standalone listening and film moments more than beat-matched edits.
Using it in a prompt
Add "expressive rubato" for solo piano or string pieces that should feel performed. Avoid it when you need to cut video exactly on the beat.