I will mix and master your song for a professional sound

Once you finish your recording you must commit the 24 plus channels of audio to one stereo track, that's mixing. Once you have that stereo track you eq it and level it in order to send it to the public, that's mastering.

A Stem is a group of tracks rendered into a stereo file. For instance, you might have several channels of drums (Kick, snare, claps, perc, etc.) that once rendered are labeled as Drums. Usually, the stems used are Drums, Bass, Synths, Leads, Guitars, Lead vocals, Background vocals, samples and FX.

Is better to leave aside reverbs and compression unless they are part of the sound. When in doubt send the dry stem and a reference stem with the fx on