Intake Valves and Exhaust Valves.
Engine valves are located in the cylinder head. The main function of the engine valves is to let air in and out of the cylinders. That air is used to help ignite the fuel which will drive the pistons up and down.
There are two types of engine valves; intake and exhaust valves.
The intake valves of course let air in, and the exhaust valves let exhaust air out. The more air you can move air in and out of the engine the more efficient, and therefor power the engine will have. This is why the engine valve plays a pretty critical role in an engines performance.
Pistons travel up and down inside cylinders. At the top of the pistons journey are the valves. There are a varying number of valves depending on the manufacturer. As the piston is at the bottom of the cylinder, the intake valve opens to let air in, it then closes so the cylinder is air tight to build compression. Once the piston goes through the compression and firing stroke, the exhaust valve will open and let the exhaust out. It then closes immediately after.
But you may ask how do the valves open and close? There is a shaft that pushes on the all the valves called a cam shaft. Be sure to check out the cam shaft link for more information on the camshaft.