Mastering is the last stage of music production where final sequencing, level, and EQ tweaks are made to try to turn a group of songs into a record. It's exacting and exhausting work, and very different now than it was when vinyl was the preferred media for storing music. I can master right here. Screw Bob Ludwig. Mastering engineers wisely encourage everybody to believe that only they can hear well enough to really do this stuff right, and they have a point.
Nothing can substitute for experience, skill, great monitoring systems, and the best equipment.
That said, here is a story to consider. A local New Haven band called the Jellyshirts made a beautiful, dynamic, varied record that they finished a few months ago. They spent two freakin' years on it, paying for it all themselves, traveling many hours to a recording studio where they like to work, and putting a lot of soul and sweat into getting it right. Pit put out Crash Land. With this record finished, on a pair of DATs, they took it to a mastering engineer recommended by their recording engineer.