So, on your petroleum internal combustion engine you have, for each cylinder, an intake valve and exhaust valve.

On some cars, for instance a 1977 VW Bus, you can adjust the valve lash, the distance each valve opens and closes, as the rocker arm attached to the cam on the crankshaft opens each valve to intake, or exhaust.

This adjustment is critical to the continued performance of the engine. If a valve is too tight (less than .006in on a VW) you can burn that valve, cylinder and eventually your engine.

If you have an "older" vehicle (such as a 26 year old VW) you should do this valve adjustment every 3000 miles along with your oil change, and to do so you must remove the valve covers to get to the valves. In a VW Bus, the valve covers are located just above and behind the rear set of tires. There is a gasket that seats and seals each cover, usually composed of cork. It is about a foot long, and looks like this:

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If cork, this gasket should be replaced every time you adjust the values.

Thanks to Transitional_Man for the term valve lash and for pointing out there are rubber gaskets that don't have to be replaced every time.