What makes a song 'explicit' in Apple's eyes....?

Solution 1:

Looking up the iTunes User Guide, it defines "Explicit" as

A label—issued by media producers—that alerts consumers to content that might be inappropriate for children.

So, it appears that they are going off the RIAA's definition of "Clean/Explicit"

Additionally, I was able to find a iTunes Users Guide for PC that goes into more detail:

The restrictions available depend on which store you use. In the U.S. iTunes Store, you can prevent anyone using your computer from previewing or buying:

• Items with a Parental Advisory Label (from the Recording Industry Association of America website)

• Movies that don’t have a specific rating (from the Motion Picture Association of America)

• TV shows that don’t have a specific rating (from the TV Parental Guidelines Monitoring Board)