What is a rock climbing cord used for?

Answer 1

Usually referred to as a rope.

Your rope is tied to a harness that goes around your waist and thighs. It is then looped through some kind of protection on the wall, (a bolt or gear or anchor chains, depending on the type of climbing you're doing) and finally the other end is looped through the harness of your buddy who is your "belayer".

His job is to act as a weight in case you fall. With this weight on both ends the rope both stops and cushions your fall. This way, you can climb up sheer rock faces and not worry so much about plummeting to your death or dismemberment.