Mirrors are just pieces of glass. They are used in many ways for example in telescopes, flashlights, headlights of cars and lamps in lighthouse.
It was discovered that glass forms when melted solid materials are cooled quickly, so that they do not produce crystals. The main ingredients for making glass are sand, soda ash, or potash and lime, melted together at a very high temperature.
Mirrors have been coated with a reflective material on the back, so that when a beam of light strikes the surface none of it is absorbed. The beam of light is actually reflected away again.
The beam of light is reflected at exactly the same angle as it struck the mirror, but in the opposite direction. This can be visualized if you imagine a snooker ball striking the cushion of the table and bouncing back at an angle.