In days gone by, cars had simpler transmissions with fewer gears, sometimes only three. It was possible to add a separate overdrive which connected between the transmission and the driveshaft. This overdrive could operate in two modes: one in which the output rotational speed was the same as the input speed, and an overdrive mode where the output speed was greater than the input speed, with corresponding decrease in output torque.
All modern transmissions have the equivalent of overdrive built into them. Whenever you are running at highway speed, you should always go into overdrive. Put your car into the highest gear. At highway speed, you don't need a lot of torque and a high gear ratio lets the engine turn over at the slowest possible speed for your traveling speed. That allows the engine to operate in a regime where it is more fuel efficient. So in highway cruise, go into overdrive to use the least gasoline for each mile!