It's a rust library for joinstr which can be used by developers to build apps for joinstr or implement coinjoin in existing wallets. They can even use bindings if they want to use another language.

Pythcoiner is working on it and its development was sponsored by nostr:nprofile1qqs94rjcrut2qyhzf54xgq2j44tzqkxtqe0pmu5wjp7ph75zc9gv3wspzemhxue69uhhyetvv9ujuurjd9kkzmpwdejhgqgjwaehxw309ac82unsd3jhqct89ejhxqg5waehxw309aex2mrp0yhxgctdw4eju6t05aupqu