Music in the public domain or under a free license
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.
https://en.wikipedia.org/wiki/Free_music