That is member THOR (non-practicing at the moment).
Valve lash, is basically setting the idle as low as it can go without stalling.
He is putting a feeler gauge between the rocker tip and valve stem (you kind of drag it through), if it goes through like it was sliding through air it's wayyy too loose. If it wont' go through it's too tight. You use a 3/8 wrench, or ratchet and socket or wobbly and socket on the adjuster screws at the push rod side of the rocker to loosen or tighten the lash (gap)....(righty-tighty/lefty loosey)....it just takes a little tweak of the wrist to change the gap a couple thou, it will be a little hard with the wrench bouncing up and down a few hundred times a minute...
The best way to go is do one set so you don't have to swap feeler gauges every cylinder, then do the other set (do all the intakes at .010, then do the exhausts at .020). The feeler gauge should have a little resistance if lashed correctly, much like a warm knife cutting through a refrigerated cube of butter.
You will notice that as the lash changes the engine may change rpm (if too tight the idle will get lumpy and may stall the engine).
Once you are completed, button up the valve cover and redial in your proper idle speed and it should be good to go.
