What do you mean by z-index in CSS?

The CSS property known as Z Index (z-index) specifies the hierarchy of overlapping HTML elements. A higher index element will be positioned above a lower index element. Note that only positioned items can use the Z index ( position:absolute , position:relative , or position:fixed ).

The stack order of an element is specified by the z-index property.

A higher stack order element is always in front of a lower stack order element.