Fill power is a measurement of how much space once ounce of down will occupy when lofted, measured in cubic inches. For example, one ounce of 850 fill power down will loft to a volume of 850 cubic inches while one ounce of 550 fill power down will only loft to 550 cubic inches. Since the warmth / temperature rating of a sleeping bag is based off how much loft there is / how much air is trapped, sleeping bags using a lower fill power down will require more down to achieve a specific temperature rating than a bag using a higher fill power down. Bags with a higher fill power like 700-850 F.P. will be lighter and more compressible than a similar bag using a lower fill power down like 550-650 F.P., if the other factors (temperature rating, size, fabrics…) are consistent.