In the context of social invitations, RSVP is a request for a response from the invited person. It is derived from the French phrase répondez s’il vous plaît, meaning “Please respond”.
The high society of England adopted French etiquette in the late 18th century, and the writings of Emily Post offered a standard of responding to invitations. Late 20th century editions building on her 1920s beginning work say that “Anyone receiving an invitation with an R.S.V.P. on it is obliged to reply….”, and some recent editions describe breaching this standard as “inexcusably rude”. (Wikipedia)