need some clarification on information provided in the video.

I am pretty sure that the entire element collapses in the sense that floating an element (like a div) actually removes it from the "content box" of the parent element. In the example that is given in this lesson, basically all the content is removed from the parent element, so it basically ceases to exist. It's not just the height, but in the example the height is the kind of notable change. In a different situation though, you might be able to see the collapse of the width as well. For example, if there were multiple boxes on the same line and you floated the content of one of them.

I believe he said what he meant to say. Guil was just saying that if all of the elements within another element (like the header container div) that containing element "collapses" down to the height that it would be as if those floated elements were no longer inside of it.

If you open the workspace and follow the changes that Guil makes, at the end you can play with the main header's CSS and see that changing the padding of the header still has the correct effect in the browser. The header just acts as if there was no content inside of it.