If you could see stacked shapes from the side, the stacking order would look like this.
Take a look at this illustration, and the concept of stacking order will be more clear to you. Although you'll never see stacked shapes from this side in Visio, imagining them like this helps you get the picture.
By default, Visio puts shapes in a stacking order as you add them to the page.
The rule to remember, for how Visio establishes the stacking order, is "The last one in wins." What that means is, the last shape you place on the page is highest in the stacking order. In this example, the computer is on the top of the other shapes because it was the last one added to the page.
But this rule doesn't always work to your advantage. For example, what if you added the computer, and then added the circle? The circle would hide the computer, and that's no help.
Obviously, you want to know how you can change the stacking order. Read on.