That is the Over Drive mode. used when traveling on the highway to get better gas mileage. I don't use mine in the city because when it goes in OD at lower speeds or when towing, it causes more strain on the engine and Transmission.
OD = overdrive
An overdrive is a device which was commonly used in automobiles to allow the choice of an extra-high overall gear ratio for high speed cruising, thus saving fuel, at the cost of less torque. Usually the final or top gear is called overdrive. Non direct drive gears increase torque multiplication at the expense of higher engine rpm and thus, decreased fuel economy.
