A stem is a group of like audio sources, mixed together, usually, so it can be dealt with downstream as one unit. Examples are, string instruments, a full orchestra, background vocals, percussion instruments, a single drum set, or any other grouping that may ease the task of the final mix.
Having stems for each song uploaded to Songtradr will save time down the line if a buyer has specific requirements and on a tight timeframe, significantly helping your chances of procuring and keeping a placement.