Nostr: What can a relay learn about my activity?

A relay receives the signed events you publish to it, the filters you request, and the connection that carries those requests.

Short answer

A relay receives the signed events you publish to it, the filters you request, and the connection that carries those requests. Public events include your public key, timestamp, and content. A relay may also see your network IP unless another network layer hides it. Use relay choices and privacy tools with this visibility in mind; encryption of a message's content does not hide every request or connection.

Sources

NIP-01: Basic protocol, NIP-17: Private messages.