Torque is a measurement of rotational force. For servos, the unit of measurement is usually oz-in though some manufacturers prefer to use kg.cm. In the case of oz-in, it is the number of ounces that servo can move if the load were applied at 1 inch away from the center of the servo spline. Let's consider an example where a servo has 200 oz-in. It is able to move 200oz if the load were applied at 1" from the center. This can be a 2" pulley (since the radius of a 2" diameter pulley is 1" and the string will always feed off the pulley at a 90 degree angle), a gear with a 2" pitch diameter (again, since radius is 1"), a sprocket with a PD of 2", a wheel with a diameter of 2", or an arm with a linkage attached 1" from the center of the servo spline. If, however you took that same servo and changed where the load were applied, the weight would therefore change. For example, a 1" pulley (which has a radius of half of the unit of measurement used) would allow the servo to move 400oz whereas a 4" pulley (which has a radius of twice the unit of measurement used) would allow the servo to move only 100oz. Servo torque is most commonly logged as stall torque, or the absolute max the servo can move. It's good practice to choose a servo with at least twice the torque that your project needs so that the motor inside of the servo is not running constantly at max capacity. Running a servo at or near stall torque will cause the servo to generate heat which can lead to premature failure.