https://wiki.roblox.com/index.php?title=API:Class/GuiBase2d/AbsolutePosition
It reads “Description: The distance, in pixels, from the top left corner of the game window under the Topbar”
However, this is incorrect if you put it on a 3D GUI, but there is no where it describes the behaviour of Absolute Position for 3D objects that I can find. What I think it does is give the offset from 0,0 of the 3D GUI. This had me confused for a little while, debugging something that was working correctly.