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.
The coolant hoses are made of rubber and eventually wear out. When a coolant hose bursts, there is a huge explosion of steam from under the hood of the car or the engine simply dies. Either way, it's unpleasant and highly avoidable by changing the hoses before the fact.