Stems is the description for a single audio track within a piece of music. Usually, a piece of music consists of multiple stems that have been layered together. For example, an electric guitar could be on one stem, the bass on another, the snare drum on a third and so forth.
A music track is usually delivered in stems to a mixing engineer, who then makes the whole composition sound cohesive through mixing techniques.
Not to be confused with the Sound & Music Toolbox, where we deliver elements and editions, stems only refers to the individual instruments tracks within a composition.