This prevents the song from being copied or shared. 'Protected” means the song is DRM protected and can only be played on iTunes authorized with the Apple ID.
: A Protected AAC file was the default file type for songs purchased from the iTunes Store prior to the introduction of the Digital Rights Management (DRM)-free iTunes Plus format in April 2009. You may have downloaded it from the web or ripped the song from a CD using iTunes' built-in MP3 encoder. : This is a standard MP3 file, the classic digital audio format. Sort your iTunes songs by kind and you may find MPEG audio file, Purchased AAC audio file and Protected AAC audio file, etc.
