It's the final process of an album where all the mixes are pulled to their maximum volume so all songs sound equally loud, they are also "opened" or "thickened" with additional treble and bass to create a clear and thick sound. After that they are placed one after the other with fades if you want to and the final start/end of a song is programmed for the pressing factory. Mastering will take about 2 to 3 days and is usually practiced best in the company of fine ho's and weed smoke.
The mixing of an album this big is the killer job. We're on the very last bits. I'm being a bit anal about it.