Internal combustion engine generate large amounts of heat. "Coolant" simpy means a liquid that provides  temperature-transferring properties. In an automotive cooling system it is required to transfer heat from the engine whilst protecting the engine from corrosion (i.e. rust) and erosion which can be caused by fluid movement. Many automotive coolants also have added chemicals to extend the operating temperature range of the fluid. These are known as antifreeze/antiboil coolants.