When there’s a separate director, cameraman, and actors, the director calls “roll camera.” The cameraman then calls “speed” which started because it took mechanical cameras a couple seconds to reach proper recording speed. That’s no longer the case, but it is still the convention. Some camera operators call “rolling” to indicate the camera is recording. Then the director cues the actors with “action!” When the shot is completed, the director calls “cut.” Before cutting the camera, the cameraman lets the camera roll for a couple seconds. This additional running is called post roll, and while it is no longer necessary for technical reasons, it gives the editor more options for things like long dissolves. This sequence of commands is called “calling the roll.”
On smaller productions with just a director/cameraman it’s not necessary to call the roll. You can simply start the camera, call “action” and then “cut.”
Excerpted from Marketing With Digital Video.