Free music or libre music is music that, like free software, can freely be copied, distributed and modified for any purpose. Thus free music is either in the public domain or licensed under a free license by the artist or copyright holder themselves, often as a method of promotion. It does not mean that there should be no fee involved. The word free refers to freedom (as in free software), not to price.
The Free Music Philosophy generally encourages creators to free music using whatever language or methods they wish. A Free Music Public License (FMPL) is available for those who prefer a formal approach.
Some free music is licensed under licenses that are intended for software (like the GPL) or other writings (the GFDL). But there are also licenses especially for music and other works of art, such as EFF's Open Audio License, LinuxTag's Open Music License, the Free Art license and some of the Creative Commons Licences.
Before the advent of copyright law in the early 18th century and its subsequent application to music compositions first, all music was "free" according to the definitions used in free software or free music, since there were no copyright restrictions. In practice however, music reproduction was generally restricted to live performances and the legalities of playing other people's music was unclear in most jurisdictions. Copyright laws changed this gradually so much so that in the late 20th century, copying a few words of a musical composition or a few seconds of a sound recording, the two forms of music copyright, could be considered criminal infringement.