Radiator coolant/antifreeze has much higher boiling point and much lower freezing point than water. It also contains antirust agents. You should never use plain water in the radiator. Usually a 50/50 mix works well. There is a table printed on the container that shows the proper mix for the ambient temperature. In frigid weather, the mix is usually 60/40, coolant/antifreeze and water respectively.
Some coolant/antifreeze products come premixed as 50/50. It is also very important that when you mix it yourself, use only distilled water, not the tap water. There are too many impurities in tap water, even filtered one that it degrades the performance of coolant/antifreeze.