CSS negative z-index: what does it mean?

Question

What is the expected effect according to standards if I add a negative integer value to the element's z-index property?

I searched for the answer, but only found this:
https://developer.mozilla.org/en-US/docs/Web/CSS/z-index
"Negative values ​​to lower priority" mean nothing to me.

Answer 1

When 0 is the default z-index, an element with a negative z-index will be displayed in most elements, unless they have a lower z-index.