What does the z-index property control in CSS?

The z-index property in CSS specifically controls the stacking order of overlapping elements in a web layout. When elements with a position value of relative, absolute, fixed, or sticky are layered on top of one another, the z-index determines which element appears in front or behind the others. A higher z-index value means the element will be positioned above elements with lower values, allowing for the creation of complex visual arrangements where certain items can be emphasized in the layout. Understanding how to manipulate the z-index is crucial for achieving desired visual effects and ensuring that important elements are not obscured by others in design.