Can someone explain the use of z-index in CSS?

Question

I'm really confused

Answer 1

https://www.w3schools.com/cssref/pr_pos_z-index.asp

https://developer.mozilla.org/en-US/docs/Web/CSS/z-index

Answer 2

Z-index use for costomize elements layer

Answer 3

z-index is used to change the stacking order of positioned elements. This property works with elements that have a position property set.