Stems are a generally submixes of a particular group of tracks. The group of tracks are recorded, or "printed," into one stem, or print track (notice he placed a "p" in front of each stem.) For example, the 16 blue drum tracks (I think I counted right) in this video was submixed into one stem named pDrums. In this video, there are 8 stems (pDrums, pBass, pStrings, etc.) Stems are commonly used in remixing the original track. Hope this helps.