Light beams are reflected from surfaces in a very particular way. The
the angle of incidence is equal to the angle of reflection. This is called the
law of reflection.
Instead of the beams you see you can draw lines to predict where the light will travel. These drawn lines are called rays.
This shows how an book makes a shadow, and how you can change the size of the shadow by moving the book or the wall. Draw such diagrams carefully and you can tell what will happen.
Then the shadow is on the wall will be between where the two rays meet the screen.
Drawing rays is a way of working out what will hapen. These drawings are not pictures of what you'll see.
You can redraw the diagram to predict what will happen to the shadow.