Plato describes "The Form of the Good" in his book, The Republic, using Socrates as his mouth piece. The Form of the Good is the ideal or perfect nature of goodness, and so an absolute measure of justice. Plato also explains his theory of justice in The Republic, in relation to his conception of the ideal state, both of which necessitate rule of the rational mind; in other words, philosopher kings, who can grasp the Form of the Good.
He poetically compares the Form of the Good to the sun. Just as the sun emanates light, so the form of the good emanates truth. And just as we are able to see the world with our eyes using the light of the sun, so we can make sense of the world with our rational minds only through the assistance of truth, derived from the Form of the Good.