This is one of Plato’s better known allegories and is perfect for describing subjective realities. In this scenario, almost the entire population have spent their lives chained to a wall at the back of a cave. Their heads have limited movement so that they can only see straight ahead. Here they will see moving shadows cast on the cave wall. As far as they are concerned, this is their entire reality and they can conceive of nothing more.
One day, however, a few of the population break free of their chains and venture around the edge of the wall. Here they see a line of monks walking back and forth along the wall. These monks are carrying long poles with figures at the top of them. Behind the line of monks there is a large fire and the light from this is casting shadow onto the rear cave wall of the figures at the top of the poles. These are the moving shadows that the population sees.
Some of these breakaway people go back around the wall and tell the rest of the people what they have seen. Alas, this is too much for the people and they can’t accept any change to their reality. The returning breakaways are stoned to death for their heresy.
However, some of the breakaway people keep going beyond the line of monks and they find steps in the rock leading upwards. At the top of these steps is a big ledge where several monks are keeping the fire going. Behind the monks are some more steps and when they climb these steps, they emerge into the bright sunshine of the outside world. Some of these breakaways return to tell their story but they too are stoned for their heresy.
The breakaways are now entering a world they cannot possibly comprehend. The sun in the sky is completely outside their experience and they have no way of explaining or understanding it. Eventually their eyes would grow accustomed to the light and they would begin to see shadows and vague shapes which would gradually coalesce into the reality we all know. If any of these newly-seeing breakaways were to return to the cave now, they would be blind and the inmates would infer that the journey had damaged their eyes and be thankful they had stayed put.
Reality, it seems, is always that to which we are accustomed.