Wikipedia
In film production, a C-stand (or Century stand) is primarily used to position light modifiers, such as silks, nets, or flags, in front of light sources. The stand is constructed of metal and consists of a collapsible base, two riser columns, and a baby pin on top. In addition, a C-stand includes a gobo head and a gobo arm, which provide the ability to articulate a light modifier without moving the stand itself. The legs of C stands are designed to be nested, so many stands can be placed around a light source.
The term C-stand comes from the early history of lighting equipment where a popular sized sun reflector was 100 inches square or "century". Today the term C-stand is a popular name for the grip stand made by Matthews studio equipment and has been copied by other manufacturers.
A secondary use of C-stands is to position smaller light fixtures. A typical use would be to offset a backlight from the stand column, so that the stand itself can be placed out of shot, while the backlight hovers somewhere above the top edge of frame.