What is the significance of 650 fill power down in determining the quality and warmth of a winter jacket?

The significance of 650 fill power down in determining the quality and warmth of a winter jacket lies in its ability to provide insulation. The higher the fill power, the better the insulation, as it indicates the loftiness and fluffiness of the down. A 650 fill power down jacket will be warmer and of higher quality compared to a lower fill power jacket.