Swing width refers to the radius of the circle you create while swinging the club around your body. A lack of width creates inconsistency and a lack of power.
On your downswing, try and imagine a box resting on top of your arms as shown in the picture at right. This box is being held in place by the position of your arms, cocked wrists and the position of the club. On your downswing, imagine trying to throw this box at your ball.
The important fundamental this mental image helps portray is the need to maintain your width throughout your swing. If your backswing extends too far, you lose the width you created, and thus power. By shortening your backswing and keeping it at or below parallel, you can maintain your swing width, an adequate amount of lag and power into impact. An ideal combination to hitting the ball further.
This mental image will help you develop more power, so use it on the range and course, and start getting those extra yards.