If it is on your gearshift, It would most likely be O/D, standing for overdrive, just another term for how many times the engine spins in relation to how many times the wheels spin. It is on by default on most cars.
the propose is for power transformation, for example, if one is towing a load, one would press the button to turn it off, to put less strain on the engine, one can also turn it off when driving downhill to assist braking.
another thing it does is fuel economy, when it is on (usually), the engine will spin slower in relation to speed, using less gas.