In the following image, the "Lock Aspect Ratio" box in the left-hand column is checked. This restricts you to stretching your image proportionally. Your image will keep its form if this box is checked.
In this image, the "Lock Aspect Ratio" box in the left-hand column is unchecked. This allows you to stretch your image in any direction. By unlocking the aspect ratio, you can stretch and shrink your image, modifying its original shape.