Mastered for iTunes means that your material has been mastered exclusively for distribution on iTunes. MFiT masters need to be carried out by an Apple iTunes certified/registered mastering engineer which is technically able to adhere to Apple’s protocol. Mastered For iTunes takes advantage of the encoders that iTunes use to create the AAC format when you purchase content from the iTunes store.
The mastering will sound slightly different and usually a little closer to original premaster, with no heavy processing or aggressive compression. The final output is a sonically pleasing on-the-ears master which is thoroughly checked to make sure the intersample peaks and digital over clips are corrected, allowing the iTunes encoding process to be successful.
The files provided to you will be in 24Bit High Resolution which Apple will then convert using their Sample Rate Conversion (SRC).