z-index sets the stack order of the element.

It only works on positioned elements (position:absolute, position:relative, or position:fixed).

It's natural to assume elements with higher z-index values are in front of elements with lower z-index values, and any element with a z-index is in front of any element without a z-index.  This is how it can work, but it can also be more completx than that – see here.

position: relative;

z-index: auto;			/* Sets the stack order equal to its parents. This is default */

z-index: -1;			/* Push it to the background */

z-index: 1;