Your reflection is what you see in the mirror. Other things that bounce back at you are also reflections — light waves, sound waves, even your thoughts.
Reflection comes from the Latin reflectere, made up of the prefix re-, "back," and flectere, "to bend." So it's bending something back: your reflection in the mirror is the light waves that bounce your image back at you. When you pause for reflection — serious thinking, that is — your thoughts are bending inward. Reflection can also be a consequence of something: the way your dog mopes around when you're away is a reflection of how much he misses you.