This is a Photoflex Nylon Fabric Grid for the Large (36 x 48") Softbox. This grid limits the light spread from your softbox to 40°. It fastens to the recessed front of the softbox via touch fasteners.
Grids are essential accessories for any lighting system. A grid alters the shape and intensity of the light output from your softbox when it is mounted on a light head. Consider a grid as a painter would a brush, or a sculptor would a knife, or chisel.