The gears in a cordless drill driver serve the same purpose as the ones in a car. Putting the cordless drill driver in a low gear means it will operate at a slower speed but it will deliver more torque (turning force). Putting the cordless drill driver in a higher gear means it will operate at a higher speed but will deliver less torque.
Select a higher gear when less torque is required (when working with softer materials or smaller screws).
Select a lower gear when more torque is required (when working with tough materials or larger screws, or when drilling.)