The REV Potentiometer (REV-31-1155) converts the angular position of a shaft into an analog voltage signal. A potentiometer acts as an adjustable resistor, fluctuating resistance as the shaft is turned. As the wiper (the knob) moves up and down along the coils of the resistor and the resistance and voltage output change proportionally at each new position.
The Potentiometer has a 270° limit to rotation. The sensor detects how much rotational motion has occurred in a mechanism. A specific limit is set in code to ensure rotation stops at a certain point. This is helpful when building simple arm joints because if properly applied it can prevent a mechanism from damaging itself or other parts of the robot.