When the drill is used as a screwdriver, the numbers indicate the torque breaking point. Higher numbers mean more torque. By breaking point, I mean the point at which torque is no longer applied.
This feature is useful because you can limit the amount of torque that is applied to prevent screws, or the materials they are inserted in, from being stripped. When the screw is in all the way, you want the torque to stop.
When you use the drill as a drill, as opposed to a screwdriver, you set the number to the highest setting, In most drills that setting will be indicated by a picture of a drill bit rather than a number. It will be after the highest number. Some drills may also have a picture of a hammer. This indicates it has a percussive hammer action as well as drill mode.