A radius arm is a suspension component used to control the forward and backward movement of your suspension.
They are usually bolted to a control arm or other suspension part on one end and attached to the frame on the other end. 
The radius arms are adjusted during alignment to set toe. Where the radius arm is attached to the frame, a radius arm bushing is used to dampen suspension vibrations that would otherwise pass to the frame.
Damaged or worn out radius arm bushings may cause changes in alignment, wandering steering, clunking from the front end of the vehicle over bumps or braking, and cupping wear on your tires.