The idea here is that once someone has been philosophically enlightened, they will both know the truth of the world while also remaining an outcast to society.
In Plato's Allegory of the Cave, the story begins with multiple prisoners being shackled in such a way that their only option is to stare at the back wall of the cave. Behind the prisoners is a firelight. Between the fire and the prisoners, people walk by holding objects. These objects cast shadows onto the back wall of the cave (however the would be shadows of the people are blocked out by a wall). The prisoners have names for the shadows and this is all they know of their reality. If a prisoner were to escape, they would see the objects and the light of the fire, which would be extremely bright and harmful to the prisoner initially. As the prisoner makes is way outside, he would be nearly blinded by the light of the sun shining onto the world around him. The "light" is representative of the truth and goodness of the world. If the prisoner were to make his way back into the cave, everything would be dark and he would be blind to the old reality. The current prisoners would see this as something harmful and would go to the extent of killing anyone who dragged them out of the cave.
As we question and learn the deeper meanings of the world... the truth and all that is good... we free people from their pseudo-realities and help them reach a deeper and satisfactory understanding of the world.