Plain English Meaning
A special permission stamped on a pilot's license that says they are trained and tested to fly one particular kind of aircraft. Without it, they cannot legally be the pilot in charge of that aircraft.
Full Definition
An additional qualification added to a pilot certificate that authorizes the holder to act as pilot in command of a specific make and model of aircraft. Type ratings are required by the FAA for all large aircraft (over 12,500 pounds maximum certificated takeoff weight), all turbojet-powered airplanes, and certain other aircraft designated by the Administrator. The rating is earned through specific training and a in that aircraft type.