Treating the mix, controlling the sound, correcting problems and giving to the music its final form. It should sound right on every playback system and facilitate a pleasant listening experience at any volume.
Bringing stuff together and defining the relation between them, adding dimension, making an organic whole using high grade analog and digital tools.
Fixing problems with audio, correcting errors in performance, bringing the best out of recorded signals. A good recording is always preferable, a lot can be done though when that is not possible.