On the left you see a nave support (pillar cantonné) and the nave arcade whose thickness (2 feet) is slightly less than the diameter of the body of the pier. To the right you see the outer wall of the nave aisle and the massive buttress projecting about 4 meters beyond the outer wall.
The section reveals one of the deceptive features of Gothic. The triforium passage is not actually enclosed in the thickness of the wall, but projects out behind it. Known in French as a "porte-à-faux" this kind of overhang is a common feature of Gothic.