RSVP - Répondez, S'il Vous Plaît (in French)
which means "please reply."
The term RSVP comes from the French expression which means the person sending the invitation would like you to tell him or her whether you accept or decline the invitation. It is used in a situation when somebody running some sort of a function, it gives a way that he or she can judge the expected number of guests that will attend the function.
So, the next time you see RSVP on an invitation you receive, please call your host and respond promptly. More important, though, is the simple courtesy of responding to someone who was nice enough to invite you, even if it is to say that you regret that you will not be able to attend.