Classic road frames usually have a pair of braze-ons for the shifters on the downtube. When you combine such a frame and the so called "brifters" (brake levers with the built-in shifters), you need a pair of those. These cable stops are simply screwed onto the braze-ons so you can prolongate the brake cables to the handlebars.

They are compatible with every system, for example Shimano STI, Campagnolo Ergo or Sram. The ridges on the adjusting barrel allow you to set your derailleurs just so. Even while riding.