Top weight is extra weight the ball companies add to offset the amount of weight taken out becuase of drilling the finger and thumb holes relative to the gross weight of the ball.  Top weight also gives the ball driller a relaively close idea of how much of a weight hole you would have to drill based on the layout if you need to drill on at all.  Higher top weights mean bigger weight holes if you kick the CG out far.  Lower top weights give you greater flexibility.  Overall, I like to work with around 3 ounces of top weight cause then my options are really open there.

Good luck.