Corner mirrors are two mirrors places in 90 degree angle usually in the corner of the room. Because they are in 90 degree angle the image is reflected through both mirrors and therefore reversed twice.
I've had corner mirrors in my apartments since I was studying in Oulu university. The first ones were made from small 30x30 mirrors. Now for the first time I have a full-size corner mirror. It's still a "prototype" meaning it's not properly installed.
The left side reflection is the "normal" one-mirror reflection and the right side reflection is the two-mirror twice-reversed reflection.
Because the corner reflection is reversed twice it shows my reflection as others see me. So here I'm waving my left hand and the corner reflection also waves his left hand.
The corner mirror's reflection also always stays in the same place, in the corner. When you walk around the room, the reflection "follows" you like Mona Lisa. Always facing you and staying in the same place. No matter where you are in the room, you can always see your reflection in the corner.
Because the light always goes back to it's origin in corner mirror, it's also used in measuring the distance of the moon. The laser beam is emitted from earth to moon and targeted to corner mirror placed there by the astronauts. Because the corner mirror always reflects the beam back to it's origin, moon's distance is measured from delay of the beam getting back to earth.
Lunar Laser Ranging experiment
Here's the whole video about how corner mirror works: