What actually happens when you publish a note on Nostr?

When you write a note, your client creates a JSON object containing your message, a timestamp, your public key, and an event kind number. Your private key then signs this object cryptographically, producing a unique signature. That signed event is sent over WebSocket connections to one or more relays. The relays store it, and anyone subscribed to those relays (or querying them later) can retrieve and verify your note. There's no central server involved, just your signed data floating across relays.