It’s a French phrase, "Repondez, s’il vous plait”, and it’s hard to translate. Respond if you please is the general meaning, of course. But Respond if it pleases you or Respond if you it pleases is closer to the French – the infamous and eternally annoying French reflexive pronoun! But here’s a rough and generally reliable translation:
R. - Respond
S. - If
V. - You
P. - Please
I know how easy it is to blow off RSVP requests. We’re not all that thrilled about going to some high school or college graduation, or an inconsiderate wedding held right in the middle of our weekend, right? But in high society, one does not blow off an RSVP! It would be immediate cause for gossip, scandal and possibly grounds for ostracism.
In addition, if you cannot attend an event you've been invited to, then you're expected to let that be known in what is called Regrets. I'm forever running to the post office to send off Regrets - handwritten by the Missus on exquisite Crane & Company note cards to the constant flood of invitations that come into this house. But she's always respectful and timely in her response.
There's a new trend that a telephone number or email address appears directly below the RSVP request, eliminating the need for a hand-written note. That's an optional response, of course, but proper manners in certain segments of high society would still require a personal note, sent in the mail.
Has this been helpful? Thanks as always for reading,