A flag is an opaque panel used to block light from hitting a specific area of a scene. It can be a dedicated cinema flag mounted on a C-stand, a piece of black foam core, a sheet of duvetyne, a folded jacket, or anything else that absorbs rather than reflects light. The job is subtraction: removing illumination from a place where you do not want it, the same way a reflector adds it where you do.
Subtractive lighting often matters more than the lights themselves. A face lit by a giant softbox in a white room will still look flat because every surface bounces fill back into the shadow side. Adding a black flag opposite the key kills that bounce, deepens contrast, and rebuilds the sense of dimension that the soft source erased. Cinematographers call this technique negative fill, and it is the fastest way to make a soft key light look intentional rather than washed out.
Standard cinema flags come in named sizes: a 18×24 inch flag, a 24×36, a 4×4 floppy, an 8×8 solid. Each lives on a C-stand with a grip head, positioned between the source and either the subject or the camera. Smaller cutters and dots block tiny areas, like a hairlight bleeding onto a forehead or a kicker grazing a microphone. A French flag bolted to the camera shades the front element from oblique light that would cause flare.
In stills work, photographers often improvise. Black foam core (often called blackwrap or Cinefoil when it is the heat-resistant aluminum version) is cheap, foldable, and stiff enough to stand against a light stand. Two pieces taped at right angles make a freestanding V-flag that can carve shadow out of an environmental portrait. Product photographers use small black cards to control reflections on glossy surfaces, which is really the same idea at a smaller scale.
Flagging is distinct from a grid or barn doors, which narrow a beam at the source. A flag intercepts light after it has left the source, which gives more freedom over the exact shape and edge of the shadow. The further the flag is from the light and the closer to the subject, the harder its edge becomes. Moving it back toward the source softens the transition.
Common pitfalls include letting the flag itself drift into frame, casting a sharp diagonal line across the background instead of a clean fall-off, and forgetting that ambient light in the room can refill the shadow you just dug. A useful habit is to look through the camera with the key on, then have an assistant move the flag in and out so you can see exactly what it is doing. Subtractive lighting is invisible when it works, which is why beginners overlook it for years.