A belay device is a metal piece of climbing equipment, which is used in belaying or when one climber secures the rope for the other climber as he ascends.
It is also a rappel device that is used for rappelling or sliding down the rope to reach the ground.
A belay device allows the belaying climber to hold a fall by the active climber with the friction of the rope as it runs through the device. A belay device is an essential piece of gear that every climber must own and use. It is sometimes called a BD.
Belay devices come in several shapes and sizes, including as a plate and a tube. Some belay devices like a GriGri are auto-locking devices.