We see objects in a mirror, because a mirror, when hit by particles of light called photons, reflects the photons back to us and some reach, and enter, our eyes. Photons that hit a rough surface will bounce off of the surface in a haphazard manner, while those that hit a smooth surface, such as a mirror, only bounce off of the surface at the same angle at which they hit the object. The scientific term for this phenomenon is reflection.
Not all smooth surfaces reflect photons back to us, even though, technically, they should bounce back at the same angle at which they hit the surface. This exception to the rule results, because some smooth surfaces absorb the light particles hitting them, making it impossible for them to bounce back.
Another apparent exception to this rule is that, although our bodies are rough, uneven surfaces, off of which light bounces at random angles, our images reflect off of a mirror. The reason for this apparent contradiction is simply that when we stand in front of a mirror, some, but not all, of the light particles bouncing off of us will hit the smooth surface of the mirror. The ones that do reflect our images back to our eyes at exactly the same angle at which they hit the mirror.
In other words, photons that bounce off of any part of our bodies and hit the mirror reflect back to our eyes from only one place on the mirror, and at only one angle. It follows that each point on our bodies that reflects back to our eyes from one point on the mirror produces an image in the mirror. All of the images together make up our reflections, like it or not. And remember that mirrors don't lie!