A gear train is a system of interconnected gears that work together to transmit motion and force. Gears are circular mechanical components with teeth that mesh with each other, allowing them to transfer rotational energy. By changing the size and arrangement of the gears, a gear train can increase or decrease speed and torque, making it useful in various machines. Gear trains are commonly found in devices like watches, bicycles, and automobiles. They play a crucial role in converting the input motion from a motor or pedal into the desired output motion, enabling efficient operation of the machinery involved.