It's a nostr based discovery protocol for mints. You can independently view / list mints on https://bitcoinmints.com/ and it's interoperable with how Mutiny is pulling them in too. 

Tony, you have probably address this else where but what determines what federations appear in Muntiny?

I only see a couple at the moment, how does one add their Federation?

Wumbo

lightning

Blinded Authentication Tokens in Mutiny

Rsync25

That's correct. It also helps with the receiver offline issue in general, so we prioritize that. You can always swap to your lightning channel at any time after receiving payments.