User blog comment:DancePowderer/Where Monarchs Dream, Part 1/@comment-24799733-20140511213355/@comment-1595565-20140511215315

It's the standard reply given after receiving an invitation to an event, usually a party or conference. It's French and stands for Rondez-vous S'il Vous Plait. It indicates to the host whether or not you're going. If you've ever received a party invitation, then you've RSVPed.