Device inside a pinball machine that makes a unique sound to indicate that a free game has been won.
How it works: Electricity is applied to a solenoid coil for a split second, creating a momentary electromagnetic field. The magnetism pulls a metal plunger (often with a hard plastic tip) through the coil. After approximately one inch of travel, the tip of the plunger strikes a hard, fixed surface, usually either a metal plate or the wooden wall of the cabinet. A sound occurs at this moment. As soon as the electricity is removed and the magnetic field disappears, the plunger is returned to its original position by means of a spring.
The knocker is also known as the thwacker, and the accompanying sound is sometimes called (you guessed it) a thwack. Knockers on '80s and '90s pinball machines mostly used a metal plate, creating a very loud "thwack" sound, while most earlier knockers simply knock on wood, making a softer "knock".
To complicate matters further, more recent machines (late-'90s and on, presumably) use a "virtual knocker", which simulates the sound of a real thwacker using the pinball machine's digital sound system. The virtual knocker sounds pretty close to the real thing, but allows for the modern convenience of knocker volume control.