CSS negative z-index: what does it mean?

Question

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

I searched for the answer but only have found this:
https://developer.mozilla.org/en-US/docs/Web/CSS/z-index
The "Negative values to lower the priority" doesn't mean anything for me.

Answer 1

An element with a negative z-index will be rendered under most elements, unless they have an even lower z-index.