Mastering music is truly an art form. It is  a blend of technology, psychoacoustics, trained ears and musical perception. The smallest increments of compression or equalization can have a major impact on the entire track. A good mastering engineer will make subtle decisions about sonic balance, bringing out the most important frequency ranges of different aspects of a recording and achieving a blend between the bass and high frequencies that can give a normal track some character and make a good track great.