R.S.V.P. : Tell the host whether or not you plan to attend the party.

What RSVP Means

The term R.S.V.P. comes from the French expression "répondez s'il vous plaît",

meaning
"please respond".

If R.S.V.P. 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.




Why It's Inconsiderate to Not RSVP

An incomplete list of respondents can cause numerous problems for a host including difficulty in planning food quantities, issues relating to minimum guarantees with catering halls, uncertainty over the number of party favors and difficulties in planning appropriate seating, among other things.
So the next time you see R.S.V.P. on an invitation you receive, please call your host and respond promptly.