In the context of social invitations, RSVP is a request for a response from the invited person or people. It is an initialism derived from the French phrase Répondez s'il vous plaît, literally "Reply if you please" or "Please reply".
How do I find out I've been invited?
A black raven will appear on your windowsill, a paper scroll attached to his leg. You'll need to fugure out how to take it off. Everything you'll need to know will be written there. Don't forget to send your raven back.