Mastering is the process where you take the individual mixes of all the songs (not the individual TRACKS, i.e. kick, snare, guitar, vocal, etc.--that's mixing) and assemble them into one cohesive album. It involves making sure the songs are all consistent volume-wise, and also doing any final overall EQ (sometimes over the course of an album, songs are mixed by different people or on different days or in different studios, so one song might have more bottom end, one might be brighter in the top end, etc.). Mastering smooths out all these little differences, inserts the 2-second gaps between songs, etc. It's the final process to turn it all into one cohesive listening experience so you don't constantly have to get up and re-adjust your stereo between every song.
It is a totally different skill from mixing, and while I will pseudo-master demos and stuff if necessary, I prefer to leave mastering to someone else on big projects I'm involved with. It's nice to have an outside set of ears at the very end of a project, listening in a million-dollar room specially constructed to make every frequency range perfectly audible.