When the projector is at a vertical angle to the screen, the projected image's top may become wider than the bottom or vice versa. At a horizontal angle, one side of the projected image will be wider than this other. This is called "keystoning".
Keystone correction corrects this condition so the projected image has a normal shape. You should note, however, that keystone correction also compresses the image, which can cause some distortion and loss of image quality.
If image quality is the highest priority, turn off auto keystone correction and position the projector so it is pointed straight at the screen.
