They move the rear leaf springs higher to make your truck lower. Hangers lower a truck 2 inches. They attach at the front of the leaf springs, and essentially move the leaf spring mounting location up 2 inches. The kit above also shows shock extenders and bump stops. Blocks can be are placed between the axle and leaf springs to lower a vehicle further, provided that the axle is already on top of the leaf springs.
Shims are available to maintain the correct pinion angle.
Blocks are available in 1 to 4-inch sizes. Dropped leaf springs replace the stock leaf springs. The leaf springs shown are a mono-leaf design. Using dropped leaf springs usually means sacrificing some of your payload capacity. Drop leaf springs are available in 1 to 5-inch drop applications.