Its a bar mounted from your axel to the frame, and its to prevent the springs from twisting....i think? I really have been wondering how exactly this works, i know the can have shackels at the frame mounting end, and yeah i dunno haha help me out here!
under torque, the axle wants to rotate in the opposite direction as the tires are turning. Forward motion of the tires causes the rear axle/pinion to rotate up. As traction/friction changes, the torque load changes.....ever felt your tires hop? Perfect example...every time the tires hop, the pinion rotates up when traction kicks in again.
The traction bar is designed to prevent/limit axle rotation during those hop situations. See my write up for a pic, but the usual look for offroaders is a long V bar with a upper and lower axle mount and a front-to-back movable mount at the frame end (this allows for less supension travel restrictions).
Traction bars are only an issue with leaf spring vehicles....other suspension designs use links to locate the axle ane eliminate wrap.