Internal engine components are lubricated with oil. Engines have many seals and gaskets that prevent oil from leaking out of the engine. The valve cover gasket is located on the top of the engine. It prevents oil from leaking onto the hot exhaust system, which could cause a fire.
Gaskets deteriorate over time, allowing engine oil to leak. An oil leak can not only cause a fire but also can drain your engine of oil, which can cause serious engine damage.
To replace the valve cover gasket, the technician removes the bolts securing the valve cover to the engine. S/He removes and cleans the valve cover, and then replaces it.