Stem mastering is a type of mastering that requires the submission of a mix dismantled into several submixes.
For example, a song can be submitted as 5 stereo files (drums, bass, guitars, vocals, BV’s).
Once the files have been stacked on top of each other, they will give back the original mix.
The main difference here is that now the ME can surgically intervene on only where the problematic issues occur.
For example, if the problem lies in a naughty spike @200Hz, on a stereo mix, the ME will have to remove that frequency across the whole mix.
When mastering from stems, the ME can identify where the problem lies (eg. the guitars) and remove that frequency only from that stem leaving all the other instruments untouched.