They're a miracle. https://i.nostr.build/VTNSHb0ZSIYt75Zq.jpg

Think of a post office.

They do the thing

think of it this way, the verb "relay" is to pass something on, typically information, news, or signals, from one source to another. now imagine that medium 🔉 being the relay, the noun. Easy peasy.

depends who you ask

it's one of the tubes that comprises the internet

Its kinda like a server, but also not. It's connects through the internet.