Mastering is the final step to the recording process. It is a process whereby a number of songs, after being mixed down, are EQ'd, compressed as necessary, and balanced in volume with each other, so that they will sound good when placed together on a CD.
The goal of mastering is to put to final polish on the songs. It brings the tracks up to industry standard levels and prepares the album for duplication and distribution. Mastering will allow your tracks to compete with others in the industry.