What is Mastering?
Mastering is the last creative stage of the recording process and the bridge between the worlds of creative recording and duplication. When making a recording, the producer and artist typically have a picture in their minds of how they want that recording to sound to the public. 
The mastering engineer makes that possible; the sound, the time between songs, the volume of each song, the song order - we take mixes and turn them into albums or singles ready for duplication. As a new listener who is removed from the project until the mastering stage, we have the ability to hear the music we work on from a fresh, unbiased perspective. We can utilize this unique viewpoint along with our specialized skills and tools to work with the artist and producer in pursuit of their vision.
In addition to their conceptual contribution, the mastering engineer is also responsible for making sure that a recording retains its original vision and translates outside of the studio environment. When a single or album leaves the mastering studio it is ready to have identical copies made during the duplication process and then be distributed for broadcast or to the public.