The engine lubrication system consists of oil pump, oil filter, oil filler cap, oil dipstick, and oil pan.  The oil pan at the engine bottom holds the oil.  The oil pump forces the oil through the filter and internal passages of engine, and then the oil drains back the oil pan through other passages.

Oil is used to lubricate the moving parts of a engine.  Oil clean dirt and impurities from the engine into the filter and prevent them from moving back to the engine.  Oil can also remove part of the heat from the engine.  Therefore, changing engine oil frequently will prelong the engine lifetime.