Mastering gives tracks a professional polish that they may have been lacking before. You can pay professional studios for this service, or you can use software at home to get similar effects. One of the best audio recording software products designed specifically for mastering has long been Ozone, by Izotope. Through March 6, Ozone 5 is available for only $199 (regular price $249), with discounts available if you own previous versions. The full, professional suite, Ozone 5 Advanced, is available for $999.
You can read more about this software release here: http://www.izotope.com/products/audio/ozone/index.asp