A ratchet wrench is a hand tool used for tightening or loosening fasteners, such as nuts and bolts. It features a mechanism that allows the user to turn the tool in one direction while the socket remains engaged with the fastener. This design enables quick and efficient work without needing to reposition the tool after each turn. The ratchet mechanism consists of a gear and a pawl, which allows for continuous motion in one direction. Ratchet wrenches come in various sizes and types, including socket wrenches and combination wrenches, making them versatile for different tasks in mechanical and automotive applications.