When recording an album, artists sometimes use various studios, producers and engineers for different songs. The result is that each song will have its own sound, sonically. Mastering is the process of making all those songs sound coherent and part of the same album. A good mastering engineer has impeccable ears and equipment, and will correct any minor deficiencies in the mix that the mixing engineer might have missed due to the sound of his/her room. He’ll also raise the level of all the tracks so they’re “hot” (loud) and even in volume. By putting each song through the same mastering gear and adjusting to the same level, it makes them sound more like they are coming from the same album.
