DRM (Digital Rights Management) is a technology (or several technologies) to give content providers control over redistribution and access to material.
Apple's iTunes Store, as well as many e-books vendors, have adopted DRM schemes in recent times.
This article provides some details about Apple DRM:
Understanding how Apple’s FairPlay DRM works helps to answer a lot of questions: why it hasn’t been replaced with an open, interoperable DRM that anyone can use, why Apple isn’t broadly licensing FairPlay, and why the company hasn’t jumped to add DRM-free content from indie artists to iTunes.