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;