A "RootKit" is a piece of software that many viruses use to hide themselves from the Operating System and from Virus detectors. They are extremely difficult to remove and improperly removing them can cause damage to your Operating System.
Apparently when you insert a copy protected CD from SonyBMG into your computer, it presents you with a EULA, upon your agreement a RootKit is installed and the disc's contents are then viewable.
SonyBMG has been shipping out CD's with this protection for over 8 months and a rep from SonyBMG states that "Obviously there are a lot of people who don't like the technology, and we will take note if we need to." Source