Tappet clearance, also known as valve clearance, is the small gap between the rocker arm and the top of the valve stem (Yoke). Engineers can observe and adjust tappet clearance only when the valves are in a closed position and the engine is cooled at the compression stroke.
The main purpose of tappet clearance is to allow a small expansion gap of the valve stem and pushrods to accommodate the effects of a heated engine.
Note that, if this isn’t done, the valve will not sit correctly as the engine heats up. As mentioned above, this could eventually result in power loss, irregularities in the engine, and other issues.