It’s like a telephone company for nostr. You need at least one. You can have many. If you and someone else don’t share a mutual telco, it may not be possible to see each other’s notes.

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Relays are like a hub on a bicycle wheel, where all the clients are spokes, publishing and reading their messages from the hub. Except that clients can be on as many hubs as they want, creating a redundant mesh of broadcasting, or rolling down the road.

nostr21.com is a relay I run, if you want to try it.

a server that apps sends and receive messages to