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:
誰かが私がそれで何をすべきか、その機能は何かを知っていますか?
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: