The basic gist is that there are fake mp3/mpeg files circulating on peer-to-peer filesharing networks. I.e. if you use a program like LimeWire to download a music file or video clip, you may not actually get what you thought. Instead, when you try to play the file, it installs adware on your machine.
I’m sure that I’ll have several people contacting me about this tomorrow, so how bad is it?