A gear or a cogwheel is a mechanical part that has cut teeth and fits into another cogwheel to transmit rotational motion. By using gears, one can achieve a mechanical advantage by optimising the gear ratio.
In simple terms, in a gear train, if we have one gear that has a radius of 50cm, and a second gear that has a radius of 10cm. The force required to make the bigger wheel rotate will always be greater, but will also always move slower.
Energy is constant in these cases, so using cogwheels, one can transmit energy. Using gears, I can increase speed on the smaller wheel, if I haven’t got the necessary speed of the rotation on the initial power source but I have the force to turn it. Or I can generate bigger force using this system by using power source that generates the speed of rotation. Basically, cogwheels can transmit force to speed, and speed to force. Also, it can be used to change direction, since two wheels connected to each other, rotate in opposite directions.