What are these two "clips" on the bolts holding by rear disc brake bracket to the frame? My rear disc brake is attached to a bracket, which is attached to the frame.
This bracket is held by two bolts that each have a clip on them:



The clips are clamped around the bolts, and can easily be twisted around the bolt (without the bolt moving).

What are they for?
From https://www.jensonusa.com/Shimano-XT-BR-M775-Caliper-Fixing-Bolt:


  Keeps the bolt from backing out. Torq the bolt then slip the lock over the bolt head and rotate in the direction the bolt would unscrew unntil the tabs hit something. This will stop the bolt from turning if it loosens.


Also: https://bicycles.stackexchange.com/a/29154/908