To reformat a wide screen movie for a standard definition (SD) TV screen, which is more square. Prior to the advent of wide screen TVs, pan & scan was performed on many films for home viewing. Today, it is no longer required.
How It's Done
As the original movie is played, a technician decides which part of the scene is critical and moves a standard TV viewing window left or right across the wide image to capture it.
Since nearly half the original scene is missing with pan & scan, artistic elements are often degraded. For example, landscapes are always clipped, and when two important objects are at opposite ends of the frame, one will be lost. Pan & scan eliminated all or most of the "letterbox" effect (black bars at the top and bottom). See letterbox
Shoot Both Formats
In order to avoid the extra cost of pan & scan, many wide screen movies were shot with a standard TV outline in the middle of the camera's viewfinder so that the director could keep critical objects in the center of the frame at all times. In that way, the movie could be automatically converted for standard TVs with 4:3 aspect ratios by truncating both sides automatically. See anamorphic DVD
Panning the Scenes
Although occasionally the director of the movie is involved with the pan & scan, it was usually left up to a technician to move the standard TV window across the original scene and decide what should be retained.