what's The document and other elements behave like the fixed positioned element does not exist?how to understand?

Question

Fixed positioned elements are removed from the normal flow. The document and other elements behave like the fixed positioned element does not exist.

Answer 1

it appears you are currently doing the CSS3 fundamental tutorial
display:fixed;
display:auto;.
display:inherit;
display:relative;
display:float;
here is a reference that may help you understand how these and other aspects of CSS3 work.
https://www.tutorialrepublic.com/css-tutorial/

Answer 2

Can you give an example to understand what you are saying?