This is a powder coated metal spacer that lies under the tent cross rail to provide more lift when needed to clear roof mounting hardware.
In some cases if your Prinsu Or Similar brand roof rack sits low to the roof, the manufactures mounts will come very close and may in fact tap the roof causing damage. These spacer plates can be stacked as needed to create more lift forcing the angled tent brackets to rise up away from the roof edge.
These are available in .250 thick and .125 thick black metal.
You can stack them as needed.
They can be held secure using a typical thin nut and 1/4x20 button head on one side.