Key functions of coolant
Coolant helps your engine stay cool in hot weather. It keeps the engine from boiling over and also protects the engine from freezing during the winter. In other words, coolant increases the boiling point and lowers the freezing point of your engine. That's probably why people often group coolant and antifreeze together.
Other than engine temperature regulation, there is another crucial benefit to coolant: it prevents corrosion of your cooling system. Coolant has a rust inhibitor in it so that something in the water does not eat away into the metals. But there are a variety of inhibitors and, depending on the type, the color of your coolant can differ. You might have seen blue, green, red, or black coolant on the store shelves but they are not always compatible with each other. So to be on the safe side, make sure to get the same type of coolant topped off as the one you have been using.