What is a roll pan? Why buy a roll pan?
A roll pan is a rear "fascia" many truck owners buy to replace that factory metal rear bumper. The roll pan gives a truck a much more custom, clean look as compared to the bulky factory metal bumper. Of course, by replacing the factory bumper with a roll pan, you lose the functionality of using the bumper as a step.

How do you install a roll pan?
According to Wiki Answers: "You will need to take off the tailgate by first disconnecting the tailgate support cables on either side. You can usually half-open the tailgate and one side will be able to slide up and off. (See repair or owners manual for proper removal on your vehicle). Now that the tailgate is off, disconnect all wiring leading to rear bumper for license plate light, tow packages, etc. Next, remove rear bumper via the brackets attached at the frame (usually 3 or 4 bolts). This will completely remove your stock rear bumper. After completing those removal steps: With tailgate off, set roll pan into desired position making sure everything looks square to the sides and bottom. (Note: pulling or pushing of the bed corners is sometimes necessary for proper fit). Using duct tape, attach roll pan and reinstall tailgate to ensure clearance when tailgate is in operation. Adjust roll pan as necessary to obtain proper clearance and desired look. FINALLY bolt roll pan into place using supplied screws (or weld for steel roll pans if desired). Reinstall the tailgate and you are done!"