It means that you have the tendency to see the world in lines when no lines in fact exist in reality. But this doesn't actually manifest as ruler and pencil geometry.
A cube is a made up object like anything you look at is in fact understood by you as a made up object that your brain approximates based on the light refraction. The thing is though, that while the cube a platonic form, and somebody had to come up with it for the purposes of geometry, it is not a natural form. You will not interpret a chair as a cube, you'll interpret it as a hunched over animal or a bundle of branches. Cubes are not included in the shapes ancestrally encompassed in human unconscious.
Golden rule of course is a meme, something more substantial here would be unconscious understanding of natural human body proportions. Like knowing that eyes are generally in the middle of the head or that your fingers start somewhere around your crotch and the length of a palm goes from the chin to the middle of the forehead. All I'm saying is that for the most part these proportions are not truly symmetric. One side of the musculature is dominant, one eyebrow points slightly another direction, one tit is bigger than the other, you may not have noticed it, but your brain did. That's all there is to it.