These clips (there are more than one) are located on the seat stays and the chain stays (and perhaps elsewhere; I don't have my bike in front of me). Follow any cable—brake, derailleur, or speed sensor—to the back wheel and you will see that the cables are attached to the frame (to prevent them from hanging loose and potentially get tangled with wheel spokes and causing an accident) by means of a clip. Even better, the frame has been cast or formed to have a little cradle, or saddle, for the cable to sit in. The clips attach to both sides of the cradle, encircling the cable and attaching it to the frame. It's an elegant solution.