what does z-index in css? please help me?

z-index:
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..
Ex:

<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...
http://www.w3schools.com/cssref/pr_pos_z-index.asp[^]