Ken is scientifically correct, but technically wrong. The purpose of the cardboard on the grill is to reduce the amount of air flying directly through the rad.
The radiator is a big hunk of metal with a looooooong narrow tube that snakes back and forth for like 100 meters that your coolant/antifreeze goes through to cool the engine. The reason why it's placed behind the grill is so that when the car is moving, air automatically blows through it and therefore reduces the need for a fan to blow air through it.
Putting cardboard over the grill allows the liquid flowing through the rad to maintain more heat. Ludicrous in the summer, but sometimes necessary in the winter. If your car heater isn't warming the car very well, this is a pretty likely culprit (when it's minus a gazillion). If it were only minus 5 and your car isn't warm, then likely your heater core or just crappy coolant.
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