Overdrive in an automatic transmission allows the car to travel at the same speed using less power, making it more economical on fuel. By using a gear that turns the transmission faster than the engine, overdrive allows the engine to turn fewer times, saving on wear and fuel.
Most cars with automatic transmissions have overdrive as their highest gear. For four-speed transmissions, overdrive is equivalent to a fifth gear, which engages only as the engine's rpms reach a certain level in the drive, or fourth gear.
Some cars have overdrive switches, labeled O/D, which can be turned on or off, though most sources advise leaving it on at all times other than when towing a load or navigating up and down steep hills. In those situations, the overdrive switch allows the driver to disengage overdrive to keep the engine from over-revving or to allow the gears to be used for braking.