The Allegory of the Cave. Plato realizes that the general run of humankind can think, and speak, etc., without (so far as they acknowledge) any awareness of his realm of Forms. ... In the allegory, Plato likens people untutored in the Theory of Forms to prisoners chained in a cave, unable to turn their heads
The allegory of the cave is a metaphor designed to illustrate human perception, ideologies, illusions, opinions, ignorance and sensory appearances. The cave is a prison for individuals who base their knowledge based on ideologies.
The main theme of Plato's Allegory of the Cave in the Republic is that human perception cannot derive true knowledge, and instead, real knowledge can only come via philosophical reasoning. In Plato's example, prisoners live their entire lives in a cave, only able to see shadows.