Antifreeze is exactly what it sounds like. It prevents your car's thermal distribution to reach the freezing point during times of very cold weather. Coolant is essentially any mixture of water and antifreeze. You want to have about 50/50 distribution between water and antifreeze so that you have some antifreeze protection for Winter while keeping the radiator's boiling point to an optimum (to prevent too much cooling). Too much antifreeze causes your radiator to overheat because this lessens the ease for the radiator to carry heat (hence, overheating).
Basically, what you do is fill your radiator tank with a mixture of let's say a half-gallon of antifreeze and a half-gallon of water. You should use distilled water because you want to prevent as many minerals from accumulating in there that'll cause deposits later.
Hope that helped.