Light, obviously, is an essential part of photography, which means "light writing" in Greek.
While we spend a lot of time and energy on putting light into a photo, there are times when you want to keep light out, or remove it.
For example, when you want to keep light from your light source, be it a flash or continuous light, from spilling over onto the background or into your lens.
(Unless you're J.J. Abrams, then knock yourself out with that lens flare.)
One way to do that is with a flag, basically a black device that you can put on or next to a light to keep it from spilling onto a background or a wall and scattering back into the photo.
You can buy flags in a variety of sizes, some requiring stands to use. But you can make your own for a fraction of the price, using velcro and craft foam.
Youtuber Spyros Heniadis demonstrates how much of a difference a 6-inch-by-9-inch piece of foam attached to a flash head can make. His method allows you to vary the size of the flag and its shape to best sculpt your light.
With the low cost, you can make flags for all your flashes. You could also use it for continuous lights, but I would recommend doing that directly on the light only if you're using LED or CFL bulbs, as incandescent or halogen bulbs would get too hot.
