It can create, for example, a pop-out video overlay, which can be moved around and resized. It then sits over top of the other content on the page (and remains over other tabs) and will remain fixed as you scroll down the page and similar. On a site like YouTube, you could pop the video out and proceed to scroll down and interact with comments while the video plays to the side or above.

https://support.mozilla.org/en-US/kb/about-picture-picture-f...