RSVP...I know it's French but what does it mean?

The term “RSVP” on an invitation is an acronym for a French phrase, repondez s'il vous plait, meaning “respond if you please.” It is a polite way of requesting a response to the invitation, not a suggestion to the invited guest to respond if he or she feels like it. Some invitations stamped RSVP include a RSVP card and an envelope to make responding even easier. The phrase “response requested,” which means essentially the same thing, may also be used.
If a party is being held locally, the invitation may be the first that you hear of it. However, if you live in a distant location and your presence would be genuinely appreciated, you should be aware of the date and location of the party before you receive the invitation, giving you plenty of time to make traveling plans. This is sometimes indicated by a “save the date” card, which is sent out before a formal party invitation to make guests aware of the upcoming event. Invitations to important events like weddings, graduations, and initiations are also sent out to distant family members as a courtesy, to alert them to the event, but these guests are not always expected to attend.
If you receive an invitation to an event which you do not plan to attend, and the invitation includes a RSVP card, it is polite to check “not attending,” and to include a brief note expressing your regrets. If no response card is included, call the phone number included on the invitation or mail a brief note in response, expressing your regrets. You do not need to delve into your reasons for not being able to attend, but you should respond within three days of receiving the invitation, so that the hosts know not to expect your presence.
If the invitation is to an event which you plan to attend, fill out the RSVP card to indicate this, and include information about who will be accompanying you. You should do this promptly, ideally within three days. Take care to read the invitation carefully, as it will provide information about who is being invited along with you. “Ms. Jones and friend” or "Ms. Jones and guest" suggests that Ms. Jones and another individual are invited. “Mr. Smith and family” suggests that Mr. Smith, his partner, and his children are invited, while “Mr. and Mrs. Smith” suggests that the children are not invited. In the above instance, Ms Jones would note that she planned to attend the event along with Ms. Dunham, and would express her delight at having been invited.
Filling out an RSVP card is a basic and important courtesy. The party hosts collect RSVP cards so that they know how many guests to expect, which will help them to make seating and catering arrangements. In addition, if hosts are inviting you to stay at a hotel or resort during the event, sending an RSVP card ensures that they will set a room aside for you. An RSVP card can also be used to convey important information, such as “my child is allergic to nuts” so that your hosts can be warned ahead of time.