This is were another pair of ears, a woman / man with tonnes more experience, with a vast discography and a great, great studio comes in! (to save the day so to speak!)
The mastering engineer is most commonly a person who wasn't involved in the project from the start. She/ he will bring a fresh perspective.
It is the person who will bring the whole project (usually, or traditionally as it was, an album - a collection of songs-) together.
She/ he will give the album a holistic sound, and will try to make it stand out, reinforcing its strong points and correcting any weaknesses.
During the last decade, where the loudness wars were more intense, the traditional role of the mastering engineer deviated a little bit and not always to the benefit of the songs! This was not necessarily the Mastering engineers say or intention usually. The process often was (and to certain situations still is) a competition of making the song louder than someone else's record!
Something to the detriment of the music itself unfortunately and against the artists own hard work, undoing any musicality that was there in song...Hopefully this trend it will stop sooner rather than later, actually there are signs that this practice is abating...
Mastering:
Mastering is a very delicate procedure; Needs world class equipment in a world class sounding studio.
I'd only do it with projects that I have not involved closely in the recording or mixing stage. Its always better to have a different set of ears.
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