A tappet is a mechanical device used in engines to adjust the valve timing. It is typically made of metal and is connected to the camshaft, which is a rotating shaft that operates the valves. The camshaft is driven by the crankshaft, which is connected to the pistons. The tappet is responsible for adjusting the valve timing so that the engine runs smoothly and efficiently. CAM is the three-letter acronym for camshaft.