Mixing and mastering are sequential steps in music post-production, and they require different skills, tools, and listening perspectives.
MIXING works with the multitrack session — all the individual recorded tracks (vocals, drums, bass, synths, etc.). The mixing engineer adjusts levels, panning, EQ, compression, reverb, and other effects to make all elements work together as a cohesive whole. The output is a stereo mixdown.
MASTERING works with the stereo mixdown. The mastering engineer applies final EQ, compression, stereo enhancement, and loudness optimization to prepare the track for distribution. They ensure it sounds good on all playback systems and meets platform loudness standards.
A common analogy: mixing is building and furnishing a house; mastering is landscaping and painting the exterior.