Well, everyone keyed up on your "make my mixes come to life" statement, no one really answered your first question: What is mastering? It's basically putting your collection of songs together into an album. Adjusting levels and tonal balance so the album has cohesion from one song to the next. Setting pauses between songs, editing starts and endings, fade-ins and fade-outs, PQ codes and redbook standards for CD's (whatever that is??).
The mastering engineer can also make corrections to a mix in a macro setting, such as was the case with my collection of tunes where I had a little too much low end energy. But your mixes should already have life, breath and depth before going to a mastering studio.
Peace,