RSVP is an abbreviation of the French phrase "Répondez s'il vous plaît," which translates to "Please respond."  In invitations or event announcements, this expression is usually used to invite guests and inform the organizers whether they plan to attend the event or not. Thus, an invitation may contain the text "RSVP by (date)" to remind about the need to respond by a specific date. This allows organizers to understand the number of participants and prepare for the event. This is a very important aspect of planning certain events.