Fill power tells you about the fullness of the down in your duvet. Down with a high fill power is more lofty, more insulative and recovers its fullness more easily. Good quality down has a higher fill power rating and lasts longer. Shake out your duvet every morning to help revive those precious down clusters and keep them working hard for you.
Fill weight is simply the weight of the fill inside a duvet. If weight is refered to as "gsm", then it refers to the weight of the fill within a square metre.