what is the use of z-index in css code example

Example 1: css z index

img {
  position: absolute; 
  /*position: relative;
  //position: fixed;*/
  left: 0px;
  top: 0px;
  z-index: -1;
}

The z-index property specifies the stack order of an element.

Example 2: default z index

the default z-index is 0.