The prisoner's eyes are used to the dark inside the cave and so at first, the sunlight is blinding. Eventually, their eyes adjust to the light and they begin to see reality, as opposed to the shadows that they always thought were reality. This was meant to symbolize philosophy and the path to enlightenment. When you first begin questioning your perception and the world around you, the answers you come across can be confusing and even overwhelming. However, if you continue to search for the truth, eventually it will become easier and you will see things clearly.
The escaped prisoner, meant to represent a philosopher, returns to the cave to try and tell the other prisoners what he has seen. They refuse to believe him and tell him if he releases them from their chains they will try to kill him. This is meant to show how fiercely people cling to what is familiar to them, and how you cannot force someone to question things. They have to become ready to do so in their own time.