It's a rhythm game. This repository is technically the future version Osu! know as Osu!Lazer but It's going to replace Osu! in not so long time now. It's the official repo and it's (I think) compatible with Linux since this is net core.
It's open source, license MIT.
As this task has been marked Needs Maintainer for a month with nobody having stepped up to become maintainer, in addition to providing an acceptable patch for inclusion, marking as WONTFIX. Feel free to re-open when a patch has been submitted via the proper processes.