Frets on Fire is, according to the Wikipedia, a music video game, a clone of the Guitar Hero video games, in which the player emulates playing a guitar with the keyboard of their computer.
Miriam Ruiz blogs about it:
The game included some non-DFSG-free song files and some internal fonts which needed to be replaced in order for the game to enter Debian repositories. Theres a pack of new songs made by Carlos Viola Iborra and released under a free license. Thanks Carlos!