DRM basically protects the files from being ported over to other devices/cd's etc.
Its really annoying, and so without special software on an MP3 player for instance, you acan't play files you've downloaded. Luckily alot of MP3 players support DRM, but it still leaks power from devices, etc getting the licences.
They need to quit that to be honest. Its not illegal to want to have your music with you. They shouldn't restrict you just because you might send it to mates, etc (which wouldn't work with DRM)