This diagram is designed to help show you why the tilt angle of the panel is important and how to optimize your solar panel.
First of all, the sunís rays go out in all directions, but because the earth is so small and far from the sun, the rays are basically all parallel.
In Figure 1 you can see how the earth sits on its axis at an angle. In this example the northern hemisphere would be having winter, meaning the sun sits lower in the sky and you would want your panel more upright facing the sun (see Fig.2). During the summer months the sun is higher in the sky so you would benefit from having your panel tilted more parallel with the ground to optimize that sunlight (see Fig.3).
Finding your latitude (just type "latitude+zip" into google) will tell you what to adjust your tilt to on your panel. The tilt should be equal to your latitude plus 15 degrees in the winter and minus 15 degrees in the summer.