What is mastering?
It is what happens after all the hard work of pre production, full production, full completion of a song, the details, fill ins, perfecting & refining is done. When the artists have a final piece of work that is satisfactory enough to put a smile on everyone's face in the studio, then for professional release purposes, mastering is the service that is sought out.
This is when the song or album project is sent off for a mastering process that will be the foundation product from which all duplications for distribution will be made. At this point, all the enhancements, levels, EQ, etc, can be tweaked to perfection. Doing this task in a especially treated mastering studio allows for a very accurate listening of the music dynamics without too much acoustics bouncing all over the place.
So in a nutshell mastering is a creative art process that gets audio performances ready for mass reproductions on digital outlets, cds, mp3, or even old school cassette tape or vinyl if requested. The result of this audio profession produces high quality playback on just about all sound systems & speakers types.