The white rectangle in the blue box is your screen. Lens shift allows you to shift your screen in the designated blue region without physically moving the projector. So for example you can move one time the height of your screen up or down as fits your need, this is vertical lens shift. The same applies horizontally, only this time it's only half the width of the screen. If you want to do both at the same time you can watch this on the calculator and see you are more limited, the blue lines being the limits of how far you can go.