Z-Order is the order of precedence that the shapes in the drawing area have over one another.  This basically means which shape is on top, the shape below it, and which is on bottom.  The Z-Order area of the Arrange ribbon tab has four options:

• Bring to Front: This option will move a selected shape to the front of the Z-Order (on top of all other objects)
• Send to Back: This option will move a selected shape to the back of the Z-Order (behind all other objects)
• Bring Forward: This option will send the shape one level up in the Z-order
• Send Backward: This option will send the shape one level down in the Z-order