Internal combustion engines are often cooled by circulating a liquid known as engine coolant. The coolant in a typical automobile radiator consists of a mixture of an Antifreeze and water. The function of coolant is to absorb the excess heat from the engine parts and carry it via passageways and tubes to the radiator where the excess heat is released to the atmosphere.
Water alone is not used as a coolant since it boils at 100 Deg C and freezes at 0 Deg C. Adding Antifreeze elevates the boiling point as well and reduces the freezing point. Hence the coolant is able to stay liquid at subzero temperatures. As well as avoid boiling at high temperatures.