Fill power is the ability of a down-filled sleeping bag to spring back to shape after compression. Usually, an ounce of down contains millions of filaments, which spring back to shape when you compress them. By so doing, the filaments hold air, which keeps you warm. A down sleeping pillow with a higher fill power is more resistant to compression, traps more air, and lofts better. It, therefore, feels warmer and the manufacturer does not have to use a lot of down to keep you warm.