Fill power is a measure of how well the down in your topper recovers its fullness. A high fill power rating is an excellent indication of the level of quality and size of the down. Down with a high fill power is more lofty, more insulative and recovers its fullness more easily, it will also last longer than lower fill power down if looked after well.
There are two measures of fill power - the US measure and the EU measure (sometimes called the Lorch measure). If you're comparing fill power between products, make sure you compare like with like and check which fill power measure is being used; otherwise, you may end up buying an inferior product.
No matter what the fill power of your topper, you can keep those precious clusters working hard for you by regularly shaking out your topper to revive the clusters.