Mastering
Mastering is the process of finishing a recording for final distribution. Tracks are put in order and the transition times between the various songs or pieces are set. The volume or loudness, as well as the tonal balance of the tracks, are adjusted to provide continuity throughout the CD on playback. Other effects and processes may also be applied as needed to resolve specific issues or provide a pleasing effect.
Simply put, the mastering process produces that professional sheen to your tracks, making them sound full and rich on radio or CD, and across the various sound systems they will be heard through.