What is DRM?
DRM (Digital Rights Management) are access control technologies that limit the usage of digital content and devices (hence it is also called digital restrictions management). Those in favour of it claim it is required to fight copyright infringement. Opponents complain it limits the righful "buyer" in using the bought goods, as one e.g. can not easily read a DRM'd book obtained with Amazon's Kindle on a different device, using a reader application of own choice. Another issue is that works can become permanently inaccessible if the DRM scheme changes or if the service is discontinued (OpenSource.Com).
What does Android have to do with that?
A justified question, considering Android claims to be Open Source. But that does not apply to all components (e.g. some hardware drivers) and apps. Especially several audio and video apps (such as Google Play Music or Google Play Movies & TV, but also Google Play Books make use of DRM. As do many of the shop-bound ebook-reader, like Amazon's kindle-for-android, Barnes & Noble's nook-for-android, and many others.