The z-order controls how overlapping objects are displayed on a page. It's called the "z-order" because it deals with how objects are arranged along a page's Z axis.
In Scrapbook MAX!, changing the z-order involves moving individual objects along the Z axis—either "forward" (further from the page) or "back" (closer to the page). You can do this easily by selecting an object and using the z-order tools in the menu under Edit > Arrange.
Objects that are further from the page ("in front") are displayed on top of objects that are closer to the page. Objects that are closer to the page ("further back") are displayed behind objects that are further from the page. Depending on the objects in question, an object in the foreground can completely or partially hide any objects that are "behind" it. (Transparent parts of an image, for example, will let other objects "show through.")