The most influential attempt to systematically answer this question came out of the Tübingen School. The Tübingen School did not create a one clear interpretation but various interpretations that agreed on main themes. For a good book on the Tübingen reconstruction of Plato's unwritten doctrines see:

Plato and the Foundations of Metaphysics: A Work on the Theory of the Principles and Unwritten Doctrines of Plato with a Collection of the Fundamental Documents by Hans Joachim Kramer, who is a professor of philosophy at the University of Tübingen

Here is my too brief summary of a summary of the content of the unwritten doctrines from Dimitry Nikulin's "Plato: Testimonia et Fragmenta": There were two main principles that are the cause of all things. "The One" (ἕν) and "The Indefinite Dyad" (ἀόριστος δυάς). From these principles come a hierarchy of being: ideas, ideal numbers, mathematical entities and physical things. This profusion constitutes three realms of being. (ideal entities, intermediate mathematical entities, and physical things). Nikulin says that Plato's Parmenides is the key dialogue for a systematic elaboration of the inner-Academic doctrine of the two principles of the one and many. These principles are not in themselves caused and are the ultimate causes of other things, both ontologically and logically. All things to be and to be known must be causally reduced to or deduced from these principles.

For a more apt response that can better explain the content of the unwritten doctrines and summarize the influence of the Tübingen interpretation in the history of thought, consider submitting this question to: r/askphilosophy