The story starts from Plato the famous student of Socrates who proposed
that the concepts, people, places, and things we intuit and perceive with our senses are merely shadows of the truth. He dubbed this idea of a true thing a Form.
To explain Forms, Plato used what's become known as the allegory of the cave. In this allegory, there exists a people that are bound inside a deep, dark cave. These cave people are chained in such a way that they can only ever see a blank wall of the cave that receives light from the opening. When an animal walks by the opening, a shadow is projected onto the interior wall that the cave dwellers see. To the cave dwellers, these shadows are the real thing. When a lion walks by, they point at the shadow of the lion and exclaim, "Run for cover!" But it is really only a shadow of the real Form, the lion itself...Read more about it in the Platonic Model section of the article.