This is the big daddy of all the "Seven Magic Words."  It is super, super important - numero uno.

Grab a paper plate from your kitchen (with your parents permission, of course).  Draw a big circle in the center of the plate.  While looking at the circle, tilt the plate away from you until the circle looks like this:

 

This is foreshortening at work. When you foreshorten an object, you make one part appear closer to you than the other.

Draw a square in the center of another plate.  Now, make the edge of the square look closer by tiling the plate until you see this - a foreshortened square.

 

When you draw in 3-D, you create an illusion for the eye. You distort images to make them appear to be three dimensional even though they are on a flat surface.

Foreshortening is extremely important to fuel up to your pencil power.