The 22 levels from earth to sky refer to the development of the Wish for Budhahood (Bodhichitta), from the initial intention in a baby Bodhisattva to the full Bodhichitta in the mind of a Buddha. This list of 22 comes from the Ornament of Realizations (Abhisamayalankara) by Arya Asanga which is one of the Five Great Books of Maitreya—The Future Buddha. They were taught by Maitreya to Arya Asanga in Maitreya’s heaven and then he brought them back to this world.
Geshe Michael uses two commentaries for this teaching, one by the First Panchen Lama, Lobsang Chukyi Gyeltsen and the other by Je Tsongkapa called the Golden Garland.
However, as mentioned in the beginning of the class, he did teach this one time before privately. We’re lucky to have a recording of that teaching and we’ll include a link to that teaching here for a complete commentary on all 22 metaphors for bodhichitta: