Jawaban:

RSVP is an abbreviation of the French phrase 'Repondez, s'il vous plaît'. It translates to 'Respond, if you please' or, better still, 'Respond please'. It means the sender of the letter want the reciever to reply the letter whether you can attend or not.

The phrase "RSVP, regrets only", - or simply "Regrets only", is a popular modern variation that implies "if you do not reply, that will be taken as an acceptance". So it means don't answer the letter if you want to attend, and answer the letter if you can't attend