That means that when you fold a paper into a cylinder , light travels along the folded path . Perception gets curved along with the objects of perception , resulting in apparent flatness. And that's good enough for physicists , since they can't get out of the universe , or the cylinder .
An easy way to understand this is : Imagine you were turned upside-down (ignoring gravitational effects) . How could you tell that you got turned upside down? By looking at the rest of the world, that is still right-side up. But, if you look at your hand , it still looks unchanged from when you were normal-oriented . You can't tell if you're upside down by looking at your hand . Now image you couldn't look at the rest of the world . You could only look at yourself . Could you tell if you were upside-down or ringside-up ? Upside down with respect to what? Would it even matter? That's the situation in Riemanian Geometry .