What is meant by the term valve lead in the motor industry?

The term "Valve Lead" best describes the amount in degrees in which a valve will open before its desired stroke begins The amount of Valve lead can be influential in the amount of air being drawn into the engine. If the inlet valve is opened before the piston has reached TDC on the exhaust stroke it will increase the amount of air going in for the induction stroke. This is also true if the Exhaust valve is opened before BDC on the power stroke the exhaust gases will get a head start in leaving the cylinder allowing more space for inlet gases to enter.