The Z - Index allows the layering of objects. For example the objects; A , B, C, each have differing Z - Indexes. The objects with the lower Z-Index remain in the background in context to the other objects sharing a particular space.
In practice, Web links should have a Z-Index of 0 so that they can be located by the mouse.
The red line has a Z-Index of 0, the Sales Order has been given a Z-Index of -1. Therefore the red line rests "on top" of the Sale Order.