The z-index CSS property is a dimension that refers to the 'stacking order' of positioned elements that may overlap one another. Z-index applies to elements that have a position value other than static (relative, absolute, fixed, or sticky).
Elements with a higher z-index value are positioned in front of elements with a lower z-index. If positioned elements overlap without a z-index specified, they stack according to their source order (which element is defined first in the HTML code).
Z-index can accept positive and negative values, or auto, which resets the property to its default state. It's a powerful tool for managing layout contexts on complex web interfaces.