A valve - often referred to as either an “intake” valve or an “exhaust” valve, depending on its function - is a device inside an internal combustion engine. The valve opens and closes in time with the internal rotation of the engine when it is running, as a result of the rotating action of the camshaft(s) connected (by way of the timing belt/chain) to the crankshaft. The intake valve opens at one point in the combustion process in order to allow fuel and air into the combustion chamber to be burned. The exhaust valve opens, in turn, to allow exhaust gases to escape the cylinder after the completed combustion event and make their way into the exhaust pipe.
A valve looks something like a disc on the end of a long, thin rod (the “valve stem”), and it is typically made of a hardened steel that allows it to be exposed to extreme temperature and compression. On a modern overhead cam engine, there can be as many as thirty two valves.