Nostr: What is the difference between read and write relays?

Short answer

Write relays are where you generally publish your events. Read relays are where you expect to receive mentions or other events addressed to you. NIP-65 provides markers for these roles in your relay-list event. Some relays can serve both roles. If the choices are wrong or unavailable, people may have difficulty finding your posts or delivering replies.

Sources

NIP-65: Relay lists.