RSVP stands for French phrase "Repondez s'il vous plait" which means "please respond". It is often written at the end of an invitation card or sent as a RSVP card along with an invitation card that a person sends to invite guests to attend the event such as marriage ceremony, birthday party or any other event.
The host organizing the event wants to know whether you will attend the event or not. In simple words, he wants the confirmation to his invitation in a polite way.
It helps the host to guess the expected number of guests attending the party. The host can better plan the food, drink and other arrangements for the guests.
It is also a courtesy of responding to someone who is nice enough to invite you. So, if you see RSVP on an invitation card, call your host and confirm him whether you will be coming or not to attend the event.