A flag is an opaque surface positioned to block light from reaching a selected area. Common set flags use black fabric stretched over a frame. Cutters and floppies are related forms used for different coverage and placement needs.
Cutting light
Place a flag between a source and the area to be protected, while keeping the desired light path open. The definition of the cut depends on the apparent source size and the relative positions of source, flag and receiving surface. With the same broad source, moving the flag closer to the receiving surface generally makes its edge more distinct.
Practical example
A key light may illuminate both an actor and a picture frame behind them. A flag can cut the frame while leaving the actor lit. Check the widest camera position and the actor’s full movement, because an apparently clean cut at one mark can cross their face at another.
Flagging, negative fill and nets
A black solid can also reduce returning room light when used as negative fill. That is a different placement and purpose from cutting a direct beam. A net or scrim transmits part of the light; a solid flag is intended to block it. Choose and secure the support for the actual position, especially where a large fabric surface can catch air.
Related terms
Spill · Negative Fill · Hard Light · Soft Light · Grid
References
Matthews: flags, cutters and floppies · ARRI: grip accessories for light control