Mastering is the stage where the mixed two track master is artistically enhanced and technically prepared to be sent for manufacture. It is the final technical and creative stage in a project and the last opportunity to fix any technical and artistic shortcomings.
A good mastering job will enhance your mix with tangible qualities such as volume, tone and impact. You will also find that it can bring forward many more intangible qualities such as perceived size, space, depth, width , dynamics, groove, liveliness, drive and sheen. It can add new harmonic content giving warmth, body, musicality and even a sense of new atmosphere.When audio is passed through the very same analog equipment as many familiar classic recordings, familiarity and even a "classic quality" can be given to a project.
On a technical level Mastering will balance the sound of recordings, made at different studios or from recordings made over extended periods of time. Tracks that had previously sounded very different can be made more naturally flowing throughout an album.