Plato ends The Republic on a surprising note. Having defined justice and established it as the greatest good, he banishes poets from his city. Poets, he claims, appeal to the basest part of the soul by imitating unjust inclinations. By encouraging us to indulge ignoble emotions in sympathy with the characters we hear about, poetry encourages us to indulge these emotions in life. Poetry, in sum, makes us unjust. In closing, Plato relates the myth of Er, which describes the trajectory of a soul after death. Just souls are rewarded for one thousand years, while unjust ones are punished for the same amount of time. Each soul then must choose its next life.
As move from top to bottom you find more reality and more knowledge. For example suppose you only know the shadow of a horse, in that are at the bottom (A). The shadow has very little reality—it depends on the horse casting the shadow—and it provides little knowledge. If you now see an actual horse you have moved up one level (B). You know more about horses and the actual horse has more reality than its shadow. If you move further upward to (C) you are in the realm of understanding, the realm of mathematical ideas. Finally as you proceed upward you arrive at the world of forms (D), the highest of which is the form of the good.