It is a standard of how one thing talks to another thing. It is JSON with some fingerprinting/hashing send over Websocket. Thats basically it. What you do with it, remains up to your implementation.

That helped me understand the protocol better: https://www.youtube.com/watch?v=Tbt3jL1Ms0w

This also helps understand the whole basic concept: https://github.com/nostr-protocol/nips/blob/master/01.md