Mastering is the term most commonly used to refer to the process of taking an audio mix and preparing it for distribution. There are several considerations in this process: unifying the sound of a record, maintaining consistency across an album, and preparing for distribution.
How mastering impacts the sound of the record
One goal of mastering is to correct mix balance issues and enhance particular sonic characteristics, taking a good mix (usually in the form of a stereo file) and putting the final touches on it. This can involve adjusting levels and general “sweetening” of the mix. Think of it as the difference between a good-sounding mix and a professional-sounding, finished master.
This process can involve adding broad equalization, applying compression, limiting, etc. This is often actually referred to as “premastering” in the world of LP and CD replication, but let’s refer to it as mastering for simplicity.
How mastering can impart consistency across an album
Consideration also has to be made for how the individual tracks work together when played one after another in an album sequence. Is there a consistent sound? Are the levels matched? Does the collection have a common “character” and play back evenly so that the listener doesn’t have to adjust the volume?
This process is generally included in the previous step, with the additional evaluation of how individual tracks sound in sequence and in relation to each other. This doesn’t mean that you simply make one preset and use it on all your tracks so that they have a consistent sound. Instead, the goal is to reconcile the differences between tracks while maintaining (or even enhancing) the character of each of them, which will most likely mean different settings for different tracks.
Preparation for distribution
The final step usually involves preparing the song or sequence of songs for download, manufacturing and/or duplication/replication. This step varies depending on the intended delivery format. In the case of a CD, it can mean converting to 16 bit/44.1 kHz audio through resampling and/or dithering, and setting track indexes, track gaps, PQ codes, and other CD-specific markings. For web-centered distribution, you might need to adjust the levels to prepare for conversion to AAC, MP3 or hi-resolution files and include the required metadata.
Mastering before and after
Here are two examples from different genres, both pre and post mastering, to illustrate the sonic differences that can result from the aesthetic choices made by different mastering engineers. The first is "Promise You" by Field Study, and the second example is "Shiver" by Sarah Blacker (remix).