Basically a relay is someone’s computer that they make available for apps connect to and ask for the latest content. They can talk to each other to share content. You can run one yourself or use other people’s.

There are some great YouTube videos that explain more, and if you wanna go to the source and are technical you can check their GitHubs

With this you can run your own relay easily, you can connect to other people’s relays easily. It’s like an ocean, and you can swim wherever you want. With Twitter, parties like turkey or China or other governments tell Elon “you need to do this” and he has to. That doesn’t work here.

Did you read the Nostr terms of service?