It's reducing that your high speed and low torque motor turns to low speed and high torque. It's generally using areas where you need high torque for instance ships v.s. (A carrier connects the centers of the two gears and rotates to carry one gear, called the planet gear, around the other, called the sun gear. The planet and sun gears mesh so that their pitch circles roll without slip. A point on the pitch circle of the planet gear traces an epicycloid curve.)