"Form of the Good", or more literally "the idea of the good" (ἡ τοῦ ἀγαθοῦ ἰδέα) is a concept in the philosophy of Plato. The definition of the Good is a perfect, eternal, and changeless Form, existing outside space and time.
Thus, for Plato, knowledge is justified, true belief. Since truth is objective, our knowledge of true propositions must be about real things. According to Plato, these real things are Forms. Their nature is such that the only mode by which we can know them is rationality.