The valves are set up to open and close at exactly the right moment. One lets the fuel mixture in and closes. After the fuel explodes and pushes the piston down, the other valve lets the exhaust out. The valves are usually held in an upright position by the valve stem. The valve stem is the long straight side of the valve, like the stem of a flower.
Holes are bored in the cylinder head for the valve stems.
Worn valve guides allow oil to enter the combustion chamber and cause blue smoke in the exhaust. The valve springs keep the valves closed tightly against their seats until the valve is opened by the cam. The valve seal is a unit that goes over the end of the valve stem. It keeps excess oil from getting between the valve guide and the valve stem. The camshaft turns at one-half of the crankshaft speed.