The phrase is so purely French but is understood universally as a call to action.
It literally means 'please respond' or 'please reply'.
In the 1700s (when wedding invitations apparently began to emerge) most people couldn’t read or write so wedding invitations were announced usually by the Town Crier.
Which means, of course, that the entire town is invited!
Since a local wedding might be considered the social event of the year, sometimes families wished to limit the actual guests, ensuring that undesirables do not attend.
So the wealthier families who had sent out actual written invitations had the invitations typically include a card which was to be presented upon arrival at the ceremony.
Just like an event ticket.
The use of the RSVP was described in books based around the 1700s and referred to ladies attending daily social events.
Ladies who lunch?
With the emergence of social medias and personal websites written RSVP cards are now mainly used as a formal reply to Wedding invitations, significant birthday parties or graduation events.
The exact phrase of répondez s'il vous plait has almost universally been replaced with just its letters RSVP.
Most never question what the little letters stand for but understand the action they are calling for.