Stems are the individual ensembles or tracks of a song. Say, an artist will release his stems of one of his popular track for others to remix.
His stem could consist of (for example) a drum track which contains every drums and percs, a bass track, 1 or 2 lead tracks, etc.
Some artists / DJs will use these stems to mix only portions of one track with another, or to recreate the track live in another way.