This is a Hot Racing CNC machined anodized aluminum adjustable motor mount for the Traxxas X-Maxx.
1x Two-Piece Adjustable Motor Mount with Replaceable Lock Nut
Lock nuts are to secure fasteners or shaft with threads.
A lock nut comes with a nylon insert that conforms to the threads of the screw.
The nylon insert grips against the threads and reduces the chances of loosening due to the vibration.
For best performance use a new lock nut.