R.S.V.P. stands for a French phrase, “répondez, s’il vous plaît,” meaning “please reply.” The person sending you the invitation would like to know whether you accept or decline his or her invitation. Etiquette rules that are followed in Western cultures state that if you receive a formal, written invitation, you should reply promptly, better if that same day. For hosts planning a dinner party, a wedding or a reception, this is quite important from a practical point of view, because they need to determine how many people to count on and how much food and drink to buy.
More important is the simple courtesy of responding to somebody who was nice enough to invite you, even if it is to say that you regret that you will not be able to attend.