In this video, Jonathan talks a lot about CSS positioning, and some other basic CSS. He explains that the browser window where you see the HTML page is basically called a "viewport". He explains how to make an image or other HTML things stay where it is as you scroll down, so that it will scroll down with you. That can be very useful for making navigation bars. Also, he talks about how to use the padding, so you can position your HTML element almost wherever you want!