Coolant is a mixture of antifreeze and water that is pumped around the engine to keep it cool. The coolant itself is cooled as it passes through the radiator. Antifreeze is mainly added to prevent the water from freezing in the winter, but it also acts as an anti- corrosive to protect the radiator from rust.
If the level of coolant falls too low, then the engine can overheat, causing potentially serious damage. Although it's normal to top the levels up occasionally, if you find that you have to top up frequently, have your car inspected in the garage, as you may have a coolant leak.
Check the coolant level once a week, and before any long journey. As the coolant gradually loses its anti-corrosive properties, you should also flush the radiator and replace the coolant every two years; usually during a major service. However, if you notice the coolant has a dirty or rusty appearance, replace it straightaway.