Gears are mechanical components that transmit rotation and power between 2 or more shafts. They are used to produce a change in torque through their gear ratio. Gears can change the high speed and low torque produced by something such as an electric motor into a lower speed but with increased toque.
Gears are also able to change the direction of power, an example of this would be a car differential that has to rotate the power provided by the drive shaft 90 degrees to be able to supply power to the wheels.