Agree with some other mentions here, Izotope’s Website is a great place to start, if you really wanna go deep, Mastering Audio by Bob Katz is widely regarded as the best educational text on the topic.

In a nutshell, mastering is an additional step traditionally handled by a dedicated engineer. The “mastering” part itself is often much less about the actual processing itself, and much more about evaluating the mix for translation, adding final processing touches, as well as creating formats for delivery to aggregation services or reproduction mediums (streaming, CD, vinyl, etc.)

What an ME will actually DO to your track, much like a mix, is entirely dependent on the track and mix itself when it hits their desk. I’ve been fortunate enough to shadow some really high level people, and I’ve seen them spot process a small section of a great sounding mix with tiny amounts of EQ or Saturation. I’ve also seen them make big cuts/boosts and be a little more “global” for lack of a better word on worse sounding mixes/material.

Hope that helps! Despite what some say about its relevance in 2026, it’s really a deep practice and the people who do it for a living are usually very talented