A reflection can be seen in water, in a mirror, in glass, or in a shiny surface.  An object and its reflection have the same shape and size, but the figures face in opposite directions. 
In a mirror, for example, right and left are switched.  

Let's look at another example:

Are you getting the idea?
Let's try another...

And one final example....

Reflection in a Shiny Surface

In mathematics, the reflection of an object is called its image.  If the original object was labeled with letters, such as polygon ABCD, the image may be labeled with the same letters followed by a prime symbol, A'B'C'D'.

The line (where a mirror may be placed) is called the line of reflection.  The distance from a point to the line of reflection is the same as the distance from the point's image to the line of reflection.
A reflection can be thought of as a "flipping" of an object over the line of reflection.