A gear train is a mechanical system that consists of two or more gears working together to transmit motion and force. Gears are toothed wheels that mesh with each other, allowing them to transfer rotational energy from one gear to another. By changing the size and number of teeth on the gears, a gear train can alter the speed and torque of the output motion. Gear trains are commonly used in various machines and devices, such as watches, bicycles, and automobiles. They help improve efficiency and control in mechanical systems, making it easier to perform tasks that require different speeds or forces.