As "Doc" said, mastering is post-mixdown treatment of your tracks, to acheive a polished and consistent sound before committing the mix to vinyl/cd/whatever media. This typically involves multi-band compression/limiting and EQ, applied to the whole mix, rather than individual elements such as the drums, guitar, vocals etc, and also takes into account the limitations of the media the music is destined for - for example, vinyl has physical limitations which can result in a record which will skip on playback, and badly "mastered" tracks can even damage the cutting head during production. "Loudness" is generally sought after these days, and is acheived by the use of limiting, but comes at the expense of dynamic range. If you compare the waveforms of a good classical music recording, and a contemporary pop record, you can see just what damage can be done to your music when you try to get it "as loud as possible". So yeah, mixing and mastering is an art in itself, it's best left to a professional, but you better have a good sound engineer in the first place to acheive a good recording and a balanced mixdown, because, as they say, you can't polish a turd.