The cooling system removes the heat generated by the engine, absorbing it into a heat absorbing liquid (antifreeze) that is circulating in side the engine.
In order for the thermostat to control the minimum and maximum operating temperature of the engine. Accomplishing this allows the engine in your car to operate more efficiently.
Engine coolant is a mixture of water and antifreeze/ that when mixed properly raises the boiling point and lowers freezing point of the mixture. A typical mixture is a 50/50 solution of water and antifreeze/coolant.