RSVP is a request for an invited guest to reply back.
When the invitations are sent out, at times the host would like to know how many people are definitely coming to the event, so that they can prepare according. This is what RSVP is used for. RSVP is a request for an invited guest to reply back, to either confirm that they are coming or to confirm that they are not coming.
RSVP stands for “Répondez s’il vous plait.” The French phrase roughly translates to “Reply if it pleases you” or “Reply please”.
If the host has sent out invitations asking for RSVP, then it is considered extremely rude not to reply. Many times, people misunderstand the concept and reply only to confirm that they are coming. If they are unable to go, they do not reply, which is considered rude because the host would like to know that you cannot make it.