It goes on top of your jack when you are jacking up the vehicle. The slot on the other end straddles the pinch weld (at rocker panel) at the jacking points: http://www.wk2jeeps.com/Misc/2011_wk_lift_points.pdf
It keeps the jack from scratching the paint and slightly helps prevent dents on the rocker panels.
Don't jack it under the aluminum differential, suspension, engine cradle, etc. These Jeeps are from Fra-gee-lee, Italy (remember The Christmas Story?)