Leveling
Leveling in mastering can take one or two directions. Usually the audio levels are taken care of in the mix track by track. This is also an important aspect to mastering. Having a good mix to work with makes the job a lot easier. Sometimes a mastering engineer can receive a bad mix of a song an request that the artist have it remixed.
All that needs to be done for leveling is you put all faders completely down, an then starting with the kick drum then snare, start raising the volumes on all tracks one by one. The goal here is that as you are raising each track, you want to make sure that you give each sound it's proper volume level. You can't just have all faders set to 0db an expect the mix to be level in that way. The master fader is the main slider that stays at 0 decibel.
Each sound or instrument commands its own level to sit well in the track in it's pan position so it does not over power the entire song. A good example of bad leveling is with that classic SNL comedy sketch with will ferrel and too much cowbell. That cowbell was the star of the show.
Seriously, it's easy to realize without complication that leveling all the sounds is a key phase before the song is even touched by a mastering process. Not even compression would probably salvage a song from too much cowbell.