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.
This is a pressure plate for the Isuzu FTR model truck. The pressure plate, as its name implies, applies pressure to the clutch disc for the transfer of torque to the transmission. The pressure plate, when coupled with the clutch disc and flywheel, makes and breaks the flow of power from the engine to the transmission.