A belay device is a mechanical tool used in climbing to control the rope during belaying, which is the process of managing the rope for a climber. It helps to catch a climber in case of a fall and allows for smooth rope handling while ascending or descending. Common types of belay devices include tube devices, auto-locking devices, and figure-eight devices. These devices work by creating friction against the rope, which helps to slow down or stop a climber's fall. Proper use of a belay device is essential for safety in climbing, making it a crucial piece of equipment for both climbers and their belayers.