Complex sentence: A mirror does not actually reverse left and right any more than it reverses top and bottom; it reverses forward and backward; that is the parts of the image are reversed in the direction perpendicular to the mirror surface.
Split into simpler sentences: A mirror does not actually reverse left and right any more than it reverses top and bottom. It reverses forward and backward; that is, the parts of the image are reversed in the direction perpendicular to the mirror surface.