Light rays bounce off everything trees , houses, cars, and people including yourself. When you stand in front of a mirror, light rays bounce off your body and strike the mirror. Mirrors are made up of layers of glass. Now usually , when light travels, it will move in a straight line until it hits something.
However, when light hits a mirror, it bounce or reflects, because the mirror’s surface is smooth and shiny. These light rays come back and strike your eyes. Since light rays are invisible, you can not see them, so your brain is tricked into thinking that the rays have come directly to you without being reflected and that a copy of your self is standing behind the mirror.This copy is your image.
However, since a mirror reflects the light rays back in the same direction as in which they have come , you left and right sides in the mirror will be the opposite of what they are when you are facing forward.