If your engine isn't cool it will soon overheat and stop working. The damage caused by an overheated engine can be very serious indeed and cause major damage.
That's why it's vital to periodically, at least once a week, check the level and colour of the coolant. Your coolant should be the same colour as you put it in if it is turning a rusty colour you should definitely change it.
It's important that water isn't poured into the radiator as proper coolant is designed specifically for engines, won't cause rust and contains anti-freeze which means it works effectively all year round.