This term means that a proprietary game has been rewritten completely from scratch as a Free Software project.

Usually, the game principle stays the same, but has its own codebase, and different graphics and music. Sometimes names are changed, too, if the original names were trademarked. After some time, the Free Software clones usually surpass the original game in almost every aspect.

This is a great way for a fan community to push forward their favorite game long after the producers lost their (commercial) interest in it.

This is also some kind of sustainability, as it ensures the game won't die with the hardware it was produced for.