Anyway, the object was a tailgate extender. We weren't sure exactly how it worked or how to connect it to the truck, but after some guidance from our friend YouTube, Adam felt confident that he could connect it, and became even more convinced of its usefulness! A couple of small connecting pieces had to be ordered, but once they arrived, Adam had no trouble installing the new contraption:

With the tailgate closed, it holds all his fishing supplies in one place so they don't slide around back there. And with the tailgate open, it can be easily flipped over (without disconnecting), to keep larger loads in place. (No photo of that - sorry.) How cool is that?
And I think I recently stumbled upon the most useful aspect of all: carrying groceries! No longer do I worry about groceries sliding around in the back of the truck, so now Adam can occasionally do the grocery shopping for me on his way home from work, without concern of smashed eggs or cracked milk jugs. I knew there was a reason I loved that thing!