DRM usually stands for Digital Rights Management
Quoting from Wikipedia:
Digital rights management (DRM) is a class of access control technologies that are used by hardware manufacturers, publishers, copyright holders and individuals with the intent to limit the use of digital content and devices after sale. DRM is any technology that inhibits uses of digital content that are not desired or intended by the content provider. Copy protection which can be circumvented without modifying the file or device, such as serial numbers or keyfiles are not generally considered to be DRM. DRM also includes specific instances of digital works or devices.
For more details refer wikipedia article on DRM
