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.

(Source: Wiki)

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.