I've seen a lot of different answers on the Internet to this simple question.  Some were pretty technical and confusing.

A lot of people think audio mastering is only making all the songs on a CD a comparable volume level.  Yes, this is done in mastering, but it's only one of many processes, not the only process.

The simple answer – Audio mastering is applying effects to a full song mix (on the stereo/main out bus), in efforts to replicate the sonic qualities of a well mastered industry standard commercial song.

In mastering, you're adding effects to the entire song as a whole.  Effects such as compression, spectral enhancement, EQ, etc.

An example of a mastering process would be adding bass to a song you're listening to on your car stereo.  When you add bass, the entire song gets it.  You can't add bass to only the vocal track.  This is comparable to a mastering EQ process because it affects the entire song.