stage of post-production. It will make your track sound as good as it can by making it punchier, more transparent, deeper, wider, tighter and more controlled, fatter, glossier, etc. Sometimes there is even no need to make visible changes but to make the song sound as good as it can on any sound system.
Proper mastering is a key
stage before broadcasting the track to mass media
Apart from what has been said above, the aim of album mastering is to make individual tracks sound coherent and work as one-peace product. This means the music will flow naturally, avoiding any disturbance when song changes from one to another, the levels in all songs will fit just right avoiding the need to use any sound controls while listening to an album.