Typical cameras have lenses that produce rectilinear images.
These stretch the image so that vertical, horizontal and diagonal lines that we perceive as being straight are reproduced as straight lines.
This command distorts the image to a cylindrical projection of up to + or - 90° rotation about the vertical.
The amount of rotation can be set with the slider control in this command. Most swing lens/rotating panoramic cameras produce this kind of image.
As an example imagine a horizontal straight line stretching from horizon to horizon at some height above ground.
Stand some distance away from and face head on to the line.
The line in front of you will be seen to be at some height above the ground but looking left and right, the line will appear to converge down to the horizon.
This command will project that image onto a flat surface.