In the allegory of the cave, Plato describes a group of people who are chained in a cave so that their heads can only look at the wall in front of them. Behind them is a fire. Between the fire and the chained prisoners is a low wall, and from behind it people are walking and dangling puppets over the wall. Because of the fire, the puppets cast shadows on the wall that the chained people can see. Because the people cannot turn around, they think what they see displayed on the wall is real, not shadow.
For Socrates, whose idea Plato is explaining, this is how most people experience life: mistaking shadows on the wall for the reality of the true forms of things. The fire is a false light: it represents the senses, such as sound and hearing, which deceive us. We cannot learn the true reality of things through the senses (the fire) because all it does is deceive us that shadows are reality.
Socrates envisions one of the people chained in cave dragged up into the sunlight. At first, because he is not used to it, this person will be blinded and in pain from the sun. But as his eyes adjust, he will see in the sun's light the true reality of things, not just their shadows.
This would be such a blessing to him that he would want to go back and tell his fellows. However, he would be so blinded by the darkness when he returned that the other people would decide that the sun (truth) was not something to pursue.