I hope you know that when the rays from an object fall on any mirror (concave convex or plane), an image is formed. In case of a place mirror, the object is placed in front of the mirror and image is formed behind the mirror.We call the point at which these rays fall on the mirror as the point of incidence.
The distance at which the object is placed from that of the point of incidence on the mirror is called as object distance. Similarly, the distance from the point of incidence of the mirror to where the image is formed is image distance.
Further to this, for plane mirrors, as a rule, this image, behind the mirror, is formed at a distance that is equal to the distance at which the object is placed in front of the mirror. So object distance equals image distance. This is not true for all types of mirrors, though.