Down keeps you warm by trapping body heat and preventing it from diffusing into the cold air around you. The performance of down fill is measured by 'fill power'. The numbers 550, 600, 800, for example, refer to the fill’s quality, not the amount of fill in the down jacket as is commonly thought.
Fill power is measured by the number of cubic inches that one ounce of down occupies in a climate-controlled test cylinder. The more volume occupied by the down, the greater the amount of air trapped and the greater the insulation generated.