Antifreeze, also called coolant, is a coloured fluid (usually green or red) found in a vehicles radiator. Antifreeze serves a few purposes. The most important and known is keeping the water in your radiator and engine from freezing in cold temperatures. It also keeps that same water from boiling over in the summer. Radiators are normally filled with a 50/50 mixture of antifreeze and water. The third function of antifreeze, or coolant is lubrication — it lubricates the moving parts it comes in contact with, like the water pump.