What's nostr?

nostr:nevent1qqs0dhygw20tyy7954858u8lne65v3kckttg3axak8z79j8faan78mqpr4mhxue69uhkummnw3ez6ur4vgh8wetvd3hhyer9wghxuet5qgsvc3y03v6y9dsrj6s5h24t8r78djxn7vddna7qu9eymysl5xk50kcrqsqqqpp84w25eg

What's nostr?