Meaning "without digital rights management". DRM-free music tracks can be purchased and downloaded online and are currently offered by the iTunes Store and Napster, as well as other online digital music vendors. DRM-free music may be purchased cheaper than those sold under DRM, and may also be of lesser quality. Most stores offering DRM-free music will provide users with options to upgrade the the higher quality DRM version by charging a small fee.
Also see "When Is Downloading Music on the Internet Illegal?" in the "Did You Know" section of Webopedia.