Répondez S'il Vous Plaît
It means 'please respond'. Don't believe me?
If you receive an invitation to some event, there is often a request for an RSVP. If there is one, you should respond. Even if not, you should reply just the same. First - for just general courtesy - if someone took the time to invite you, take the time to tell them if you accept or not. Second, often an RSVP will inform those planning whatever event that they need to be prepared for x number of people, then they will make sure to by food for x number of people. If you don't get back to them, they have to guess how many x will be and there might not be enough food and drink, or there may have been too many seats booked, costing more money. If you are courteous enough to respond, do what you say you will do; if you say you will be there, then you should show up, if not, then don’t!