Loft, a.k.a. Fill power is a measure of the "fluffiness" of a down product. Down (or any insulation) provides warmth by trapping a layer of air that separates the cold side from the warm side. A thicker layer of trapped air gives more insulation. The higher the loft, the more insulating air pockets the down has, meaning it will keep you warm at night. While this air layer keeps you warm, it also allows for greater breathability, ensuring a peaceful nights sleep.