From the Austin Wedding Blog:
On a wedding invitation, the term “RSVP” is an initialism, for the French phrase, repondez s’il vous plait, meaning “respond if you please.” It is the most commonly recognized way of requesting a response to a social invitation.
Filling out an RSVP card to attend a Kansas City wedding reception is a basic and important courtesy.
The reason soon-to-be-wed couples send out an RSVP is to help predict the total number of guests to expect at their wedding. This helps them in finalizing catering arrangements and seating charts.
Remember, acceptance of a social invitation does not constitute a legal obligation; your RSVP just communicates your best intentions to attend. Be kind and reply.
KC-Wedding-DJ Tip: On the other hand, if you accept an invitation, remember that the Bride & Groom will invest as much as $25-75 per person (non-refundable) on food and alcohol in response to your invitation acceptance. It would be rude and inconsiderate to simply decide not to attend.
Read more at the Austin Wedding Blog.