DRM is Digital Management Right. For iTunes, if you buy songs from the iTunes store for $0.99, those songs can only be played on max 5 computers and unlimited iPods. Those songs cannot be transfered to any devices. If you want DRM-free songs, then you can either buy/upgrade to the $1.29 version or buy from another source like Amazon MP3 etc. Songs you ripped from the your CD collections are also DRM free.
There are ways to make duplicate copies of your DRM songs, but that outside of this discussion.