What is a stem? In short, it is a categoric breakdown of a mix, in other words a sub-mix. Drums stem, guitars stem, vocals stem, etc. When they are summed back together they sound exactly the same as the final stereo mix down.
The most common practice is to create as few stems as possible. For Rock music it might be drums, bass, guitars, synths, background vocals, lead vocal and the rest. But the variables are endless and it mostly depends on the producer how they choose to export their stems.
So what is stem mastering good for?
Stem mastering is a good choice when the mix was subject to too many changes and variables. The reasons for this could be manifold. A good example might be a collaboration project, where each musician records their parts remotely and one of them is then mixing it. By the time the mix is finished everyone involved might have a bit different view on the mix and there is no producer involved. In this case it can be beneficial for the mastering engineer to be presented with stems, as he will judge the mix with objective mind and will be able to adjust the stem balance accordingly, and treat each stem separately. On the other hand this allows the engineer to completely change the identity of the mix, so this service should be well considered by the creative person supplying the stems OR extra attention should be paid to providing me with very detailed guidelines regarding mastering!
Stem mastering might be also a good way to fine-tune a mix which has a prominent problem within one of the stems. For example: there might be displeasingly prominent “esses” on lead vocals. In this case the “esses” can be removed only in the vocal stem without loosing the same frequencies in any other group of instruments, which would be much more difficult to achieve in a conventional mastering scenario.
However I must insist that it is always better to address these problems directly in the mix and provide me with the best possible sounding stereo pre-master. If you are unsure about your mix, I offer another service, which might be of interest to you called mix supervision.