So far you know x-axis and y-axis and what they refer.. Like wise there is another axis called z-axis which refer the order of elements. Also it referred as ordering of elements in stack order.
Which means element with greater z-index will appear on element with lesser z-index..
<div id="div1" style="position:absolute; z-index;100; height:100px; width:100px;"></div>
<div id="div2" style="position:absolute; z-index;200; height:100px; width:100px;"></div>
Here div2 will appear on div1...
Also z-index only works on positioned elements
(position:absolute, position:relative, or position:fixed)
Refer below for more info...