Plato believed in two worlds, the material world and the world of the Forms. The Forms differ from material objects because they are perfect and pure; while material objects are a complex mixture of imperfect properties of the Forms. According to Plato the Form of the Good is the highest reality of all. As well as being individual forms, things like truth, beauty, justice and equality also reflect the Form of Goodness. ‘Goodness’ is a quality that all the individual Forms possess. The Form of the Good links all the other Forms together.
In Plato’s analogy of the cave, the Form of the Good is represented by the sun because all of the other objects depend and get their existence from it. When the prisoner is out of the cave he is blinded by the sun’s light and the understanding of the forms and the Form of the Good is overwhelming. Just as the sun’s light allows us to see objects, the Form of the Good provides order and simplicity to allow us to gain knowledge of objects. Just as the sun provides food and growth for all living things, The Form is the Good provides order and structure which is the source of the existence of all things.
Only when you have gained true understanding of all the other forms, ie beauty, truth and justice etc, knowledge of the Form of the Good can be eventually achieved.