The term RSVP comes from the French expression "répondez s'il vous plaît", meaning "please respond". If RSVP is written on an invitation it means the invited guest must tell the host whether or not they plan to attend the party. It does not mean to respond only if you're coming, and it does not mean respond only if you're not coming (the expression "regrets only" is reserved for that instance). It means the host needs a definite head count for the planned event, and needs it by the date specified on the invitation.
RSVP = répondez s'il vous plaît