To reformat a wide screen movie for a standard definition (SD) screen, which is more square. Prior to the advent of HDTVs, pan & scan was performed on many films for home viewing. Today it is no longer required. See SDTV
How It Was Done
As the original movie was played, a technician decided which part of the scene was critical and moved an SD viewing window left or right across the wide image to capture it.
Since nearly half the original scene was missing with pan & scan, artistic elements were often degraded. For example, landscapes were always clipped, and when two important objects were at opposite ends of the frame, one was lost. Pan & scan eliminated the black bars at the top and bottom of the screen (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 was 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.