Track stems are the individual audio track files which make up a full production track.
For example, the kick, snare, hi-hat, piano(s), guitar, etc. would be sent as individual audio files within a .zip file so that you can have them professionally mixed and mastered along with your vocals.
WAV: 44.1khz, 16-bit, Mono
NOTE: These are the minimum requirements when uploading audio files for track stems. You are allowed to upload 24-bit WAV's as well.