Mastering is that last step, where you sweeten the overall sound of your track with small amounts of EQ, volume leveling, and compression/limiting.
In the case of mastering a whole album, the goal is to get the volume and the EQ of each track to sound similar (as part of an album), so as not to have eack track sound too different. You don't want one song nice and crisp and loud, and have the next track sound duller and (gasp!) quieter. An example is to compare the overal sound of an album today (overall volume as well as clarity in the highs all the way down to the lowest lows) to one that was recorded a decade or two ago. There's generally a difference.
When just mastering one track, I would suggest finding a song or two by one or two of your favorite artists that you like the sound of, then A/B compare the overall sound of your track to theirs. Tweak the compression/limiting (volume) and the EQ to get your track to sound good like their tracks.
Hope this helps.
