We see things when light bounces off them and enters our eyes. When light bounces off a surface, in science, we say it is reflected. This is called reflection of light. How light is reflected depends on the surface. Smooth surfaces reflect light relatively evenly, whereas most other surfaces scatter it in all directions. Mirrors are very smooth surfaces and reflect light in a perfectly even manner.
The diagram above shows light being reflected by a plane mirror. The red line is called the normal, which we draw vertically to the surface of the mirror to help us calculate the angles of incidence and reflection. The angle of incidence is the one formed by the normal and the ray of light that hits the surface (the incident ray). The angle of reflection is the angle formed by the normal and the light ray that is reflected (the reflected ray). The angle of incidence is always equal to the angle of reflection. The image formed in the mirror is the same size as the object and forms at the same distance as the object is from the mirror. Left becomes right and right becomes left.
A kaleidoscope is a toy used to make lots of reflections of something. When you look into it, it can look a little like this: