Carabiners are used by rockclimbers for a number of tasks, primarily connecting a rope to a piece of protection such as a nut, chock, friend, bolt, or in the old days, a piton. The protection is generally jammed or wedged into a crack in the rock. Here is an early carabiner, from 1868. This was not made for climbing, but for some other use with a rope or chain. It was probably made of steel, so would have been a little heavy to use in climbing, but it was certainly the same idea.