Valve lifters are a component which often connects a pushrod to the cam shaft. They have a larger and more round surface area than a pushrod, so it is sometimes used to improve engine performance.
Hydraulic Valve Lifters
In comparison to a solid pushrod-to-camshaft design, hydraulic valve lifters allow some compression to happen between the cam and valve movements.
These lifters require some preload to work properly. The preload keeps some pressure on the hydraulics so they don't give way too much, which would cause the valves to not open much, restricting air flow in and out of the engine.