Lash is the "slack" in the valve train system to handle expansion from heat of various parts. There is a very small gap between the valve stem top and the rocker arm when the valve is fully closed. If the lash closes up, heat caused expansion can hold a valve open, leaking compression; too loose and the valves dont flow the best amount of air/fuel or exhaust. The set screws on top of the rockers are how it is adjusted with a feeler gauge as measurement.