When positioned outside the normal flow elements, we can have overlapping elements.
To specify which element lies above the other, we use "z-index". Elements with higher z-index value will be above those with lower value.
This can be useful, if we make a headline with text below and add a background image.
The image "imageheading.jpg" is placed in the upper left corner and overlaps with the text "This is a heading".
As the image has a value of -1 for "z-index", lag behind in any overlap, and therefore remain as a background image of the text "This is a heading".