What is audio mastering?

Metroplis writes:

What is audio mastering? It is the final stage of the production process and the last opportunity to check, correct and enhance your music before it’s released to the world. Here at Metropolis Mastering we have the skills, equipment and experience to realise the potential in your recordings and take them to the next level. Our sole aim is to make your music the best it can possibly be.

Metropolis Mastering was founded in 1993 with the intention of redefining international standards.  That same passion, ambition and commitment is still the force that drives us today. Our five mastering suites were designed and built to the highest possible specification, offering the best environment for critical listening, and equipped with some...........

http://www.thisismetropolis.com/mastering