Basically mastering is the last step before you print a CD. In mastering you no longer mess with single tracks, but process the main overall track to bring in more eq or dynamics. It is important that teh dynamics of all the songs in your album sound similar without any sudden suprises, like track volume level changes or eq differences. Mastering takes care of all these. It can be done in home studios if you have a proper set up and your room acoustics is good. Rather leave it to the pros.