Plain English Meaning
A short report a pilot gives to Flight Service before a VFR flight, saying where they are going, how they are getting there, and when they expect to arrive. If they do not check in to close it on time, someone starts looking for them.
Full Definition
A flight plan filed by a pilot operating under Visual Flight Rules that records the intended route, departure and destination airports, estimated times, fuel on board, number of people aboard, and aircraft and pilot details. It is filed with Flight Service and is used primarily to trigger search and rescue if the aircraft does not arrive as planned. A VFR flight plan must be opened (activated) by the pilot after departure and closed on or before arrival.