Hot, it has to do with correct deck height to attain optimum compression ratio and engine performance.
Insufficient deck clearance causes the piston to kiss the head at high rpm and self-destruct. This problem is corrected by machining the piston top or using a thicker gasket(cheaper and quicker).
Excessive deck clearance causes a loss of compression ratio and power.This problem is corrected by machining the cylinder head's mating surface(flycutting) or using a thinner gasket(cheaper and quicker).
Deck height may be described as the distance between the piston's highest point and the cylinder head's lowest point. This is at the moment when the piston is at TDC(top dead center). This is the highest point that the piston can reach in the cylinder, at the compression or exhaust stroke.
I hope this helps.