A wheel cylinder is a component of a hydraulic drum brake system. It is located in each wheel and is usually positioned at the top of the wheel, above the shoes. Its function is to exert force onto the shoes so as to bring them into contact with the drum and stop the vehicle with friction.
It is a heavy metal plate that is controlled by springs and a lever. Its main purpose is to apply pressure to the primary clutch plate (or clutch disc), holding it against the engine flywheel. This allows energy to flow from the engine crankshaft, through the engaged clutch into the transmission / gearbox system, then through driveshafts and to the wheels.