RSVP is a French abbreviation meaning répondez s'il vous plaît, which translates as "please respond". The phrase entered English etiquette in the eighteenth century, when French was the working language of diplomacy and of letters, and it stuck as the standard request for a reply to an invitation. You will see it on wedding invitations, party invitations, dinner invitations, and formal event cards.
When a wedding card says "RSVP by 14 May", it is asking you to confirm whether you will attend by that date. A reply is not optional; silence is considered discourteous. The French original has a softer politeness ("if you would please") that the English usage preserves.