The oil filler cap is a plastic or metal cap that covers an opening into the valve cover. It allows you to add oil when the dipstick indicates that you need it.
Some cars have the crankcase vented through the filler cap.
Oil which is added through the filler passes down through openings in the head into the oil sump at the bottom of the engine. If the oil gauge gives very low readings, it means that your oil pressure has dropped. The engine would soon grind to a halt if the oil is not forced into the nooks and crannies that need to be oiled. Usually this type of engine has the camshaft mounted in the cylinder block, and the valves are opened and closed by push rods.