What Is Mastering?
Often regarded as the ‘dark art’ of audio processing, the mastering stage is easiest to think of as the final opportunity for adjustments and corrections any new music goes through before it hits your listener’s ears. And for that very reason, it’s arguably the most crucial stage.
It is not about fixing the balance of a multitrack mix, it is not about fixing any performance problems. In fact, if your mix has these types of problems, mastering should be the last thing on your mind.
While many of the same popular audio processing tools used during recording and mixing are also used in the mastering stage, they are ideally used in a much more subtle way. Whether it’s tweaking EQ, Compression, Limiting, and Stereo Imaging, the techniques applied during mastering aim to enhance as well as create a constancy throughout the audio, which is particularly important across an album worth of music.
There are literally endless pages of content online that go into far more technical detail than you could ever need about mastering (a Google search will quickly point you in the right direction). A handy source you might like to start with that provides the origins and basic concepts is https://en.wikipedia.org/wiki/Audio_mastering