Relays are servers that specialize in transmitting Nostr events (Nostr stands for "Notes and Other Stuff Transmitted By Relay"). They are the "backend" or database of all Nostr clients, so every time you create a post (or any other kind of nostr event), this action is submitted to relays. The relays store these events, and return them to Nostr cilents (websites and apps) when requested.

Relays can be really big and intended to catch as many notes/events as possible, or more niche, intended for only a specific community or purpose.

If you choose to view and interact with one relay at a time, that can essentially be considered a sub-community of Nostr. This is what you see when you browse relays on Jumble with their relay feed feature.