Also known as a Socket Wrench, a Ratchet is a device that is used to either tighten or loosen a nut or bolt. The teeth in a Ratchet means that it will only turn the nut or bolt when the device is moved in one direction. If the device is then turned in the opposite direction, it will not turn the nut or bolt. Instead it will re-set itself so that you can again turn it in the correct direction to continue turning the nut or bolt. This helps to speed up the process, without having to take the Ratchet off the nut or bolt after each rotation.
Another advantage to using a Ratchet is that they come with interchangeable socket heads, so that you don’t need to buy a complete new Ratchet for each different size needed.