The z-index is a CSS property that is used to specify the vertical stacking order of the elements on a webpage, relative to their siblings, parents, and ancestors. The z-index property takes a numeric value that can be positive, negative, or zero, and it determines the position of an element in the stack, based on its value. Elements with higher z-index values will be positioned above elements with lower z-index values, and elements with the same z-index value will be positioned according to the order in which they appear in the HTML source code, with the last element in the source code appearing on top of the other elements. The default value of the z-index property is zero, which means that the elements will be positioned according to their source order, unless they have a different z-index value specified. The z-index property is often used in web design and web development to control the visual appearance and the interactivity of the elements on a webpage, especially when the elements overlap or when they need to be layered or stacked in a specific order. Z-index is often used in overlay and readblock ads.