Mailing an RSVP card involves indicating on the card whether one plans to attend the specified event, signing one's name, then placing the card in the envelope and putting it in the mail. RSVP cards typically come with pre-addressed, stamped envelopes to make the process as simple as possible.
According to the Washington Post, increasing numbers of invited guests aren't returning their RSVP cards. When an RSVP card isn't sent back, the host of the event is unable to make accurate plans regarding catering and seating arrangements. RSVP cards often ask for information other than just whether a guest is coming to an event or not, including dinner choices, special dietary needs or transportation information.