What is a plane mirror?

A plane mirror is a mirror whose surface is planar or “flat.” The familiar mirrors found in bathrooms and dressing rooms are normally plane mirrors. The shape of a mirror’s surface affects the image seen in the mirror, as can be observed in funhouse mirrors, which distort our images in various ways.
Mirrors used in optics, as in lasers and telescopes, may be plane mirrors or convex or concave curved mirrors. Depending on the purpose, spherical, parabolic, or even cylindrical curved mirrors may be selected. The reflecting surfaces of these curved mirrors have the shapes of portions of the solids named in the description.
Differently shaped mirrors have different effects on the rays of light that strike them. One way of describing the effect of a particular mirror shape is to specify details of the image it produces. All plane mirrors produce a virtual image. This means that when an object is viewed in a plane mirror, the object appears to be behind the mirror. Furthermore, this virtual image is the same size as the reflected object, and the object and image are the same distance from the plane mirror’s surface. Images in a plane mirror are always upright, which does not seem extraordinary unless you are familiar with the various mirror and lens shapes that produce inverted (upside-down) images.
A final characteristic of images in plane mirrors is that they appear to be be left-right reversed. Most of us have noticed at some point that graphics and lettering on our clothing appear backwards when viewed in a plane mirror, and if we part our hair on the side, we may be startled to see that photographs of our faces are the reverse of what we see in the mirror.
More information on plane mirrors may be found on the page from The Physics Classroom linked below.
https://www.physicsclassroom.com/class/refln/Lesson-2/Image-Characteristics