A valve spring is a component in an internal combustion engine that helps to keep the valves closed when they are not in use. It works by exerting pressure on the valve stem, which keeps the valve tightly closed until it is opened by the camshaft. The 129768 valve spring is a specific model of valve spring that is designed to work with certain types of trucks and engines. It is important to use the correct valve spring for your truck's engine in order to maintain proper performance and prevent damage to other components.