Your factory frame is made out of "U" channel. Boxing it means welding plate steel on the open side of the "U", along the frame rails, so that you have a frame that is no longer a "U" channel- it becomes a "box" (it is now rectangular tubing, of sorts..) The stock frame was designed to flex, and it flexes a great deal- not the greatest thing for race car. Boxing the frame usually includes adding tubular cross members to stiffen the frame even further.
