Valve clearance check or tappet settings are often heard terms from mechanics; it is basically the adjustment of a cam follower (tappet), but before that let me explain you what is ‘a tappet’. It is the part that runs on the camshaft and is made to move vertically by the action of the rotating cam. In an overhead valve engine, this tappet is fitted low down in an engine block. From there, it drives a long thin push-rod, up to the top of the engine, above the cylinder head. Then there are the rockers; arranged on a rocker shaft beneath the rocker cover, reverses the direction of the valve movement to press the valves downwards to open them.
Here the cam lobe pushes the push rod which moves the tappet(the part above the valve return spring, attached to the rocker arm). If the valves are too tight the engine may experience rough running characteristics in either hot or cold or all the time. Too loose and they may produce a rattling sound which is quite unpleasing to the driver and is also likely to increase when you accelerate.
It is advised by experts to adjust tappet settings frequently whenever the cylinder head is removed and, get them checked every 3000 km as they stretch slowly and you would likely get tighter valves which results in burning of the valves (especially the exhaust valve) and then they start leaking hot gases when they are too tight. Frequent adjustment of tappets allow you to check a faulty valve and replace them or else you are expected to pay for heavy repairs.
The distance is set by using SHIMS (a thin piece of metal used to adjust the distance between two moving parts) of various thickness, they are inserted between the gaps and you are required to adjust the screw until it matches the thickness of the shim.