Brake caliper

The metal backing plates of the pads grind and wear on their mounting points and cause grooves to form, especially when do-it-yourselfers don't use high-temperature brake grease on those points. The pads can catch and bind on those spots, and they will cause clicking when the brakes are applied. Those parts are stainless steel inserts that prevent that wear from occurring. You may never notice if one of them is missing but typically there will be a single light clunk each time the brakes are applied. If only one insert is lost, put the remaining one in front so the pads are forced into it when braking when going forward. That's where it will do the most good and prevent the most wear. You can always find a replacement in a salvage yard and install it later.