Apple Digital Masters is a new initiative that combines all of the Mastered for iTunes music into one global catalog (via Billboard).
Apple Digital Masters
First launching in 2012, Mastered for iTunes gave guidelines and free software tools to music engineers that let them upload their music encode from high-resolution masters.
Apple notes that the majority of top releases on Apple Music currently are Apple Digital Masters, with about 75% of of the Top 100 in the U.S. and 71% of the Top 100 globally created under the program.
The songs are identical to the lossless audio from the masters. You can read more about the program here [PDF].