Short and sweet:

Nostr - Relays

Anyone can run a relay, but not everyone has to. Relays are servers that save and broadcast your Nostr posts/contents to clients. Relays do not talk to other relays and only forward content to clients that request data from them. Relays can limit who can post to the server and who can request data from it.

https://bitcoinconsulting.us/blog/nostr-overview-twitter-but-not