Mastering is the finishing touches given to a song. It is important to master a song so it is up to par with every other song in the industry. It is even more important to have an outsider do the mastering because they will not be influenced by anything that happened during the mixing or recording of the single/album.

Mastering means I will not, and cannot, do any drastic changes to the song. I can only find the minor flaws and fix them before release.

For albums, mastering also means I make each song flow into each other better. if song1 is very bassy while song2 is not, my job is to make them less contrasting and feel like they are meant to be in the same album.