Z-Index is a CSS property that controls the vertical stacking order of elements that overlap on a web page. When multiple elements occupy the same space, the z-index value determines which one appears on top. Elements with a higher z-index are displayed above those with a lower value. This property only works on elements that have a position value other than "static" (such as relative, absolute, or fixed). Proper use of z-index helps manage the visual layering of content, ensuring important elements remain visible as intended.