Coolant, which is also known as antifreeze, is typically a mixture of water and ethylene glycol. Coolant both lowers the freezing point and raises the boiling point of fluids, which is essential to preserving an engine in both cold and hot climates.
In addition to managing the temperature of your car's engine, coolant also has corrosion inhibitors and lubricants mixed in. Corrosion inhibitors help protect the car's aluminum radiator because the coolant flows through electrochemically incompatible metals like iron and solder in the rest of the engine. Lubricants in the coolant helps maintain the seals in the water pump and hoses.
Since coolant circulates widely through the engine, it gets dirty quickly and it is not uncommon for coolant to be black, sludgy and have rust floating around in it. Coolant also breaks down over time, so its ability to keep the engine cool, provide corrosion protection and seal leaks diminishes after a while. Old coolant often becomes more viscous and more difficult for the water pump to circulate through the engine. Flushing your coolant on a regular basis removes gunk and mineral deposits from your engine while prolonging the life of your water pump, heater core, thermostat and other components tied to the cooling system.