its the splash pan/air diverter for the radiator, if you live in a hot climate your truck will run a lot cooler with it installed.
here is what I did, in this picture you can see the pan trimmed for the frame horns
I cut a piece of thin abs (its actually kydex) using the UPPER grille closeout (with the latch) as the rough pattern then trimmed it to fit and screwed it down with some sheelmetal screws
thats a chevrolet, gmc is a little different, but the same plan, trim to fit then plastic.
the rad sits so high in the swap that you want to force air to go through the core. if there is any room to get around (or below) the rad, it will.