The Theory of Forms is the philosophical idea in the school of Metaphysics proposing that reality (as in the physical world) does not exists as humans perceive it, and is instead only represented to us in the form of Ideas, or Notions of true essence. The things we see, hear, feel, taste, touch and smell are simply imitations of non-physical and eternal -things- that exists outside the realms of time and space. In “Plato’s Allegory of the Cave” A human being that has spent his life facing the wall of a cave and only seeing the shadows from the outside world cast reflections upon that wall, would believe that he shadows themselves were all of reality.
Funny enough, slightly earlier, the famed Pythagoras suggest that he was onto a similar idea, but instead of the true forms of our physical world being representations of some eternal force or essence, he believed that objects and sensations were merely physical representations of some eternal code of numbers (like the matrix).