What does this piece do that came with my disc brake? I recently ordered hydraulic Shimano disc brakes with rotors and they came with a bunch of hardware, including this piece:


It looks like it would fit under the screws that secure the rotor to the hub:


Does anyone know what I should do with it or what its function is?
These things are called tightening plates or lock washers. Your suggestion was righ in where it goes, here's an exploded diagram and installed photo: