We like to think of mastering as the final polish of your song before it gets heard. Technically, mastering is the process in post-production that balances the sonic elements of a stereo mix, and it optimizes playback across all systems and media formats. A more simple way to say it is this: mastering gets the most usable volume for your song by shaping the EQ compression and stereo image. Take a listen to a song with and without mastering: