Stems are submixes of a song's individual elements, exported as separate audio files. A typical stem set might include: vocals, drums, bass, melodic instruments, and effects/FX.
Stems are different from multitracks. Multitracks are the raw individual recordings (every single mic, every single instrument). Stems are grouped submixes — the drum stem contains all drum tracks bounced together.
Stems are used for remixing, DJ sets, sync licensing (film/TV needs to adjust the mix), live performance (backing tracks), and sampling. On SpatialGig, producers sell stem packs so other artists can remix, sample, or build on their instrumentals.