Simple Definition:

RSVP means to respond to an invitation [a request to attend an event] with an indication of whether or not you will attend.

For example:

• Jane received an invitation to her friend’s wedding and RSVP’d that she would be attending.
• John forgot to RSVP to the party and missed out on the festivities.

Very Simple Definition:

RSVP means to let someone know if you are going to an event or not.

For example:

• Emily got an invitation in the mail and she texted back to RSVP that she was coming to the party.
• Mike didn’t RSVP to the wedding and didn’t show up at all.