what is the little button on the drivers side of the gear shift? it says 0/D

OD is short for overdrive which is the highest gear for relatively flat land driving on the highway. If you are driving up a mountain pass, you can push the button, the OD off light will turn on in the dash cluster, and the transmission will stay in the lower gears. That way the transmission will not overheat from shifting in and out of overdrive frequently. OD off will use more fuel, and should be used only when the transmission is frequently shifting the higher gears which is called gear hunting such as ascending a steep grade.