What does it mean to break out of a document stream in CSS?

If an element is out of the document flow, is it just a display of the break? Is it also out of the HTML?
I use JS to take this element of the parent node of the childnodes can also fetch this element;
At the same time, is the parentnode of this element still not the parent node in HTML?

Out-of-document flow is just an explanation of the parsing of HTML documents. Out-of-document flow is relative to normal document flow. Normal document flow is the HTML document structure that we do not control with CSS styles, and the order in which you write the interface is the order in which the pages are displayed. For example, write 5 div blocks. The normal flow of documents is to display the 5 div blocks in turn. From left to right, top-down order. Out-of-document flow means that the location it displays and the document code are not necessarily consistent. For example, you can use CSS control to display the last div block in the first div block. This is only the browser processing scheme. However, the DOM structure has not changed. is still the same as the HTML document you wrote. Use JS to take this node can be taken.

What does it mean to break out of a document stream in CSS?