It's a commentary on free will. You believe that you are free because your life is the only context you know. If a person would be born and raised in a prison, without contact with the outside world, his entire belief structure would be based on his environment and his perception of it much like Plato's "Allegory of the Cave." Would this person not fee that he is free? Now expand the walls of the prison to the outermost reaches of the universe. Only the "size"of the prison and the nature of the rules have changed. Do you still feel free? Would a far-away observer looking upon your "universe" not conclude the same thing that you concluded about the prisoner?