RSVP are the initial letters of a French phrase meaning "Please respond". You'll typically see these letters printed at the bottom of formal social invitations, such as wedding invitations.
If someone has RSVP'd, it means that they have replied to the invitation, either accepting or declining it.
As you can see, we've turned this abbreviation into a past tense verb ( or past participle) by adding 'd. We occasionally use this non-standard suffix when we're using a set of letters or acronymn as a verb : for example you could say, "The boss wants to be CC'd into all our emails".