A panhard rod - also known as a track bar - is the ideally horizontal link that controls the sideways movement of the rear axle - just slide under the back and look for a bar an inch or two behind the rear axle - one end attaches to the axle, and the other to the frame.
When you lift the vehicle with a spring lift (coils or spacers), axle is further away from the frame, so the rod is now at an angle and pulls the axle to one side (the right - going from memory), the relocation bracket goes between the frame and the rod, lowering the end of the rod so it is horizontal once more.
Edit
Just for clarity - the relocation bracket can theoretically be used at either end of the panhard rod - I've never seen Mudchild's own, so I don't know which end he built it for.