FILLPOWER refers to the amount of down in cubic inches per ounce (28.35 grams) under certain conditions. If an ounce of down takes up 500 cubic inches of space, then it is said  have  500 Fillpower. So we can simply see it as: Fillpower is the volume of down. The Fillpower is the most important index to determine the quality of down. if the down content and weight is same,then The higher  Fillpower mean better  warmth and comfort  , and the quality of down is higher.