is the final and arguably the most important stage of the audio production process
. Without it, tracks imply don't sound professional or complete. They will tend to lack clarity and size. They won't be as loud an commercial releases and will usually not have the same level of "richness" we come to expect from pro mixes. Music mastering is also an easily overlooked aspect of the process, and is widely taken for granted. People simply don't realize that anymore more needs to be done after the song is mixed.
So what does the mastering engineer do? While some might think its a mystery, or some sort of audio sorcery, its actually not to hard to understand. A mastering engineer's job is to increase the overall loudness of a track while maintaining as much dynamic range possible. This loudness is achieved using advanced audio tools to compress, EQ, harmonically enhance, widen, and limit the track. All of these processes must be applied in the most tasteful and musical way possible. This can take many years of practice an experience, and most certainly should be left to a skilled engineer.