Picture overlapping boxes on a page. Which one sits on top is decided along the depth axis, and $\texttt{z-index}$ is the control for that axis on positioned elements.
Give an element a larger $\texttt{z-index}$ and it layers above elements with smaller values. It has nothing to do with width, height, background color, or zoom, which are governed by other properties.
So $\texttt{z-index}$ manages the front-to-back stacking order.
\[\boxed{\text{The stacking order of elements}}\]