DRM means Digital Rights Management
DRM is a way to encode music (or movie files) and thus manage user licenses. The technique allows the files to play only a certain number of times (or for a limited time). The purpose of the technique is to prevent the illegal copying of digital music, later it is also used for video material and digital books. Meanwhile, there are several DRM systems that are not all compatible so it can be that you have legally bought a book, but cannot read it on another device.
DRM: Bad for Business [YouTube] GsmH3I8rF-E [/youtube]