“Mastered for iTunes” is a certification program used by Apple to hold recordings to a certain set of standards.  Essentially, albums that bear the “Mastered for iTunes” label are mastered with sufficient headroom and encoded using a specific set of tools, supposedly ensuring maximum quality.  Most indie artists are not able to supply their own encoded files or participate directly in the MFI program, but the tools used are available from Apple, and even non-certified music might benefit from at least having the MFI process applied to verify that it will transition well to a digital format.