Digital Rights Management (DRM) is a set of technologies which copyright holders can attach to their works - most commonly music files, but also electronic videos or books, or even software. DRM restricts the way in which people can use the content, generally by restricting their ability to copy it or to use it on more than one computer. As you can imagine it is a very hot issue at the moment!
There is an article on the BBC website about the issues surrounding DRM which gives a link to a really interesting televised debate. Well worth watching if you have the time - it seems to give a very balanced view of both sides of the debate.