These terms mean the same thing in both mirror and lens setups.
Image distance is the distance from the position of the observed image to a line representing the reflective surface of the mirror. This distance is measured parallel to the principal axis (perpendicular to the reflecting surface in a plane mirror).
Object distance is the same measurement, but for the object being observed. The diagram below shows these terms as applied to a ray diagram for a plane mirror.
All images in plane mirrors are virtual. A virtual image will be located on the opposite side of the mirror as the object. The distinction between virtual and real images becomes more important when convex or concave mirrors are used.