What is this?

Can anyone tell me what this thing is? It fell of my rear brake caliper while I was changing the rear tire. I can't figure out how it fits back on. Any help is greatly appreciated.

thingy - a set on Flickr

its a part that goes on the end of the pads so they ride on a smooth surface instead of the cast. you NEED it! the rotor goes through the middle.

It's a support shim/liner. It locates on the caliper, and the notched edges of each brake pad fits over it. Basically, it's function is to prevent wear to the caliper block from the movement of the pads. You can install it without removing the wheel. Just remove the 2 pins that hold the caliper to the bracket, lift caliper, install shim, replace caliper.