Frost forms when moisture comes into contact with the cold evaporator coils in the freezer. If this frost becomes too thick, it will reduce the efficiency of your fridge. For direct cool refrigerators, you need to defrost every time the frost completely covers the defrosting guide (it's the red plastic thing sticking out of the fridge wall). No frost refrigerators don't need to be defrosted at all because they have heating elements in the freezer to melt the frost periodically.