In CSS, the z-index property is used to control the stacking order of elements on a web page. It specifies the relative stack level of an element in the z-axis, which is perpendicular to the screen or viewport. Elements with a higher z-index value are positioned above elements with lower z-index values. Possible values: number: element's stack level is set to the given value. It allows negative values. auto: It specifies that the order of the stack is equivalent to the parent, i.e., default.