The suffix “ness” means the essence of.
Plato describes “The Form of the Good”, or more literally “the idea of the good” (ἡ τοῦ ἀγαθοῦ ἰδέα, agathou), in his dialogue the Republic (508e2–3), speaking through the character of Socrates. The Sun is described in a simile as the child or offspring (ἔκγονος ekgonos) of the Form of the Good (508c-509a), in that, like the sun which makes physical objects visible and generates life on earth, the Good makes all other Forms intelligible, and in some sense provides being to all other Forms, though the Good itself exceeds being. It is an absolute measure of justice. Plato also explains his theory of justice in the Republic, in relation to his conception of a city in speech, both of which necessitate rule of the rational mind; in other words, philosopher-kings, who can grasp the Form of the Good.
Think also of plato’s metaphor of the cave, the philosopher king, unlike everybody else can see the forms of the good, not the shadows of the forms of the good. The philosopher king can see into the realm of idea and perceive the uncorrupted forms.