Think about the way a 3 dimensional object would look from the perspective of a 2 dimensional planar reality. Lets imagine a cylinder. If the plane crossed the cylinder perpendicular to its axis, the cross section it would see would be a circle. Thus the cylinder would look like a circle from that perspective, reduced by 1 dimension. If the plane crossed the cylinder parallel to its axis, the cross section of the cylinder it would see would look like a rectangle.