An element with position: absolute; is positioned relative to the nearest ancestor/parent element with a position. So if three levels up there is an element with position: relative; that element will be the one that the other elements positioning is based off of. This is in contrast to position: fixed which is positioned based on the browser viewport.
You can see the CSS for the red box is left: 10px; top: 100px;. This is relative to the nearest positioned ancestor element, which happens to also be position: absolute; in this case. You can see the effect of togging the positioning in my gif.
If you want to know which properties and what values are being applied to an element look at the Styles and Computed tabs in the dev tools. The ruler just shows you pixel width and height.