Lateral inversion means the reversal of image i.e. left the side of the object to appear right side in image and the right side of the object appear left side in the image.

Let suppose the object is placed in front of the plane mirror. Its image will be formed behind the mirror and front side of the object & front side of image will face each other. This is due to the lateral inversion.

Let consider an object be in a rectangle shape (ABCD)-

By seeing the image (A’B’C’D’) point B’ appear in the leftmost and an object point B is at right side (lateral inversion) same with all the point of the rectangle.