Valve “lash” is the technical name for the gap between the valve and tappet. The lash is critical to a healthy running engine. Too little lash and you’ll see starting problems and reliability issues. Too much lash will reduce power and potentially ruin the valve.
If you’ve just replaced the camshaft, make sure it’s properly timed. You’ll need a set of 0.004″ and 0.005″ feeler gauges, and a 9mm wrench. Intake 0.004″ and Exhaust 0.005″