The rocker shaft is a car part which can be found inside the garage at home. It is attached to the cylinder head with 5x8mm bolts. The valves on the shaft can be adjusted with a screwdriver, see this page for more information.
The rocker shaft can be worn down with use, or if the engine is improperly built/tuned. It makes a clicking sound, similar to an improperly tuned valve train, upon failure. If the rocker shaft is damaged or broken it may be replaced at the Fleetari Repair Shop. Visually; a broken rocker shaft will be slightly bent and some of the valve springs will be gone, as well as all of the valve adjustment screws.