The simplest explanation for what a shackle reversal is- it's a bracket that changes the position of the mounting point for a leaf spring in order to increase lift height.
A little more in depth- Currently your truck has a tension style shackle (shackle pivots at the bottom and leaf spring connects at the top). The shackle reversal changes this to a compression style hanger (shackle pivots at the top and leaf spring connects at bottom). This in turn creates lift height.
Why you should want a shackle reversal on your truck. Typically lift "kits" come with one of two ways to lift the rear of your truck, a block or a new set of leaf springs.
A block is a hunk of steel that is used to space the distance between your axle and leaf springs. The problem with blocks is they create a lot of leverage on your leaf springs, causing axle wrap. Axle wrap or hop happens when the vehicle is under acceleration and the leaf springs get twisted into a S shape then release causing a hop. This is super unfavorable and hard on components. If you are trying to climb a rock, pull a trailer, or take off from an intersection, you will not be happy. The other problem with blocks is they can easily fall out causing terrible accidents!
Lift kit leaf springs will be a higher spring rate than your stock springs. This results in a rough ride because they wont be able to flex or move as freely as a lower spring rate.
By using a shackle reversal you remove both those problems. Your truck will ride just like it does now, not stiffer, and not getting wheel hop all the time. The other neat thing about the shackle reversal is it wont affect your tow rating! You will be able to tow the same as you do now.