Mastering is one of those things – it’s often spoken of in hushed tones, for fear that the Wizards of Mastering Engineering might overhear and smite thee with a curse. Everybody’s told they need it, few are told why, and even fewer really know what’s going on.
In reality, it’s less mystical and magical than one would think. It’s really just a form of audio processing done to add that final polish to an album. Of course, that’s just the basics. The tools are specialized, and the engineer tends to do things you’d never dream of doing during the recording process.
Thanks to the occasional overblown media report, a lot of people look at the mastering process as simply “making it loud.” While loudness maximization is often a component, it’s not the only component, and it’s not always the goal. Sure, if you ask me to take a track and just slam it to 0db with no dynamic range, I can do that, but I’d really prefer not to. There are dozens of cheapish tools you can get to do that, too. If that’s all you want, then you can do that yourself and you really don’t need to spend the money on mastering.
Generally, it’s going to be some combination of surgical EQ, compression, limiting and possibly a little “coloration” to put that final shine on all the tracks and make the album sound consistent. Sometimes, some stereo field adjustment will be done to expand or contract the field to provide the right sound. Phase can be adjusted so there aren’t “dead spots” in the soundfield, and everything’s got to be set up so it’s listenable on a variety of systems – from your iPod to your car stereo to the boomin’ PA at a club.