Progressive Draw Distance (PDD) is a method for increasing rez times when you TP to a new location. It works by dropping your draw distance as soon as you arrive, and then gradually increasing it. In this way, initially, there are few things around you that need to be rezzed in, and also not much data to be sent from SL servers.
As the draw distance is stepped up, more of your surroundings become visible and start to rez in. This feature is very useful if you experience slow rez times, particularly in busy locations. If you find that you still have rez issues, try increasing the step time via the slider.
To confirm that PDD is in fact active and working:
See this page for documentation on Firestorm 4.6.9 (42969) and earlier.