In his allegory of the cave, Plato likens people untutored in the Theory of Forms to prisoners chained in a cave, unable to turn their heads. All they can see is the wall of the cave. Behind them burns a fire. Between the fire and the prisoners there is a parapet, along which puppeteers can walk. The puppeteers, who are behind the prisoners, hold up puppets that cast shadows on the wall of the cave. The prisoners are unable to see these puppets, the real objects, that pass behind them. What the prisoners see and hear are shadows and echoes cast by objects that they do not see. Such prisoners would mistake appearance for reality. They would think the things they see on the wall (the shadows) were real; they would know nothing of the real causes of the shadows.
Plato’s point is: the general terms of our languages are not “names” of the physical objects that we can see. They are actually names of things that we cannot see, things that we can only grasp with the mind. Our very ability to think and to speak depends on the Forms, for the terms of the language we use get their meaning by “naming” the Forms that the objects we perceive participate in.
We may acquire concepts by our perceptual experience of physical objects, but we would be mistaken if we thought that the concepts that we grasp were on the same level as the things we perceive.