A torsion bar is an alternative to a coil spring or a leaf spring used in the front suspension of many light trucks, and in some cars as well.
The torsion bar is a long rod made of spring steel. One end of the bar will be anchored to the vehicles frame. While the other end will be attached to a moving suspension part, usually a control arm. Any movement of the control arm will cause the torsion bar to twist, but the bar has a natural resistance to twisting. This provides a spring action.
You can easily adjust the torsion bars in most vehicles to either raise or lower the vehicle. Doing anything but the most minor adjustment will however decrease tire life, and will cause you to have to get a front end alignment. Most lowered Japanese pickup trucks that you see on the road will have severely "turned down" torsion bars, which is why the front wheels on these trucks seem to bend in towards the center of the truck. Tire life is extremely short on these trucks, they will often burn through a set of front tires in less than 5000 miles.