Very little, actually. It means that if you change one dimension (height or
width) of a picture (or text box or other object), the other dimension will
automatically be changed to maintain the shape and proportions of the
object. This works in the dialog when you are changing the size by changing
the percentage or the specific dimensions (in inches, centimeters, etc.),
but it has NO effect if you resize the object by dragging on a side.
Theoretically, if you drag a corner, you will resize the object
proportionally, but I find even this rather iffy.
