It is a protocol for sharing data across internet. A set of rules on how to write the data and sign it to prove it was you who created it. The protocol also says how this data should be stored and shared. It is an open protocol, so anyone can learn the rules, and anyone can use nostr applications to share data with whoever use the same rules. This protocol can be used to share data like tweets, dms, bounties, or posts, even money as lighting invoices. Anyone can setup a nostr relay, what makes nostr decentralized, which is an advantage against censorship and can improve privacy.