A camshaft is a long metal tube that has highly engineered bumps on it. Those bumps are what open and close the engine’s valves. In an overhead cam engine, the camshaft is mounted on top of the cylinder head, directly touching the valves.
The more conventional design, called overhead valve, has the camshaft in the middle of the engine block. It has to open the valve using metal sticks called pushrods. Long pushrods are less precise at opening and closing the valves, especially at high engine speeds.