A flip kit is designed to move the rear axle of your truck from being underneath the leaf spring pack to being on top of the leaf spring pack. This will generally give about six inches of lowering to the back of your truck. But that's the basic description.
To determine exactly how much a flip kit will lower your truck you can go under it with a tape measure and measure the distance from the bottom of the axle to the top of the leaf pack while it's at factory ride height. That measurement will be within 1/4 inch of the amount of lowering you will get with a flip kit.
But there are other variables. If your truck had a factory lift block in it then you need to add that amount too. So now you might be up to 7 or 8 inches of lowering which can be too much. So some flip kits compensate for this by providing replacement frame leaf hangers that drop down farther from the factory location and actually work to lift your truck back up to a reasonable height. There are also some trucks in which the factory hangers are a welded on part of the frame and not rivetted on ones which can be removed. On these trucks they make an adapater that goes in place of the leaf spring to drop it down to a lower point raising your truck up a bit.
So when you are thinking about installing a flip kit keep in mind that its not a simple as replacing a set of hangers and shackles to lower the back.