This is an ancient mantra that is related to Tara, the Mother of all Buddhas (Shyamatara), and especially to her manifestation as Green Tara. Within Tibetan Buddhism, Tārā is regarded as a Bodhisattva of compassion, and action, who manifests in female form. Tara has 21 major forms, each of which has a different color and spiritual attribute. Of these 21 forms, two are especially popular.  White Tara, who is associated with compassion and long life, and Green Tara, who is associated with enlightened activity and abundance. Aum (Om) is not really a sound, but three vibrations expressing the unity of the trinity of Gods, Brahma, Vishnu, and, Mahesh. In essence and anyone pronouncing it directly, or indirectly, remembers trinity of Gods. As such, this mantra reads: Aum (trinity/unity of Brahma, Vishnu, Mahesh) liberates. You are liberation. You are Myself.