Fill power is used to measure the warmth of the goose-down and duck-down insulation in down jackets, down sleeping bags, and down quilts. Expressed as a number, fill power measures the ability of down to trap the warm air heated by your body in a down jacket or down-insulated sleeping bag. For example, a 900 fill-power down sleeping bag will be lighter weight, more compressible, and much more expensive than if the identical sleeping bag were filled 650 fill-power down and rated for the same temperatures. This is because it takes less 900 fill-power down (by weight) to retain the same amount of warmth as 650 fill-power down. Check full article below from Section Hiker