In double progression, you work with a given weight in a given rep range, and once you hit the top of that rep range for a certain number of hard sets (one, two, or three, usually), you increase the weight. Then, if you can at least come to within a rep or two of the bottom of your rep range in your first hard set with the new, heavier weight, you work with that weight until you can hit the top of your rep range again.
