Barndoor devices alter the shape, amount, and/or color of the light output from the flash heads. The barndoor has a built-in holder for metal and color gels.
Using a barndoor set attached to the front of the reflector of your light source will allow you to control the light cast on a subject. The more leaves on a barndoor set and the longer the leaves are, the more the light 'beam' may be specifically controlled. Positioning of the leaves will determine the size of the light shaft. Rotating the leaves will determine the angle of the shaft of light. A 4-leaf barndoor provides more light control than a 2-leaf barndoor set.