Instead of one stereo file, stems split a mix into a few parts that play back in sync. Editors use stems to lower the drums under dialogue, remove a melody that clashes with a voice, or build adaptive game music that adds layers as action grows.
Stems are different from multitracks, which separate every individual instrument and microphone.
Using it in a prompt
If you plan to use stems, ask for clearly separated parts ("distinct drums, bass and melody") so each layer is useful on its own.