White Tara is referred to as "Mother of all the Buddhas." This is because she embodies the motivation that is compassion. Her whiteness "Radiant as the eternal snows in all their glory" is indicative of the selflessness -- the purity -- of this compassion but especially the undifferentiated Truth of the Dharma.
Her Mantra : OM. TARE TUTARE TURE. MAMA AYUR PUNYE JNANA PUSHTIM KURU, SWAHA
Ohm, Tahray Tootahray tooray, mahmah ahyoor poonyay jnyana pushtim kuru[-ye], Swahhah
Tibetans say : OM TARE TUTTARE TURE, MAMA AYUR JANA PUNTIN KURU SOHA
Essence of the Mantra : Increase in longevity, merit and wisdom.
In a world known as Various Lights, there was a Buddha called Dundubh-ishvara [Lord of the Sound of Drums] and he had a devotee, a princess called Jnanachandra [Wisdom Moon.] For many ages, she made offerings to him, and to the 'hearers' and bodhisattvas, until finally there arose in her the determination to, herself, become a buddha. She was advised that she would first have to seek a rebirth in a male body, for who had ever heard of a female buddha?
"Nonsense," she thought. "What difference does the form of the body matter? In fact, to dispel this incorrect notion from the minds of certain beings, I will forever be reborn as a female!"
Those who wish to attain supreme enlightenment in a man's body are many, but those who wish to serve the aims of beings in a woman's body are few indeed; therefore may I, until this world is emptied out serve the aims of beings with none but a woman's body.
Then Wisdom Moon sat determinedly in meditation for many ages. She attained the knowledge that events do not arise, and the state called Saving All Beings. Every morning before she had taken food, she introduced and fixed innumerable beings in the state of acceptance; every evening she did the same, and so she became known as Tara the Saviour.
Reborn into the realm of Buddha Amoghasiddi in the era called Vastly Extended, Tara took another vow before him: She determined to protect the sentient beings of the infinite worlds of all ten directions from harm. She settled into the state of meditation called 'Defeating all Maras,' and during the day, fixed in contemplation innumerable heavenly rulers of beings, and in the night, also those of the heaven of power of vision over others. She became known as Tara the Swift, and Tara the Heroine.
Then, in the era called Beginningless, a monk whose name was Stainless Light was empowered via the light of compassion of all the tathagathas [buddhas] and became Avalokiteshvara (Lord of the World, called in Tibetan, Chenresi). In him, two lights emanating from all the buddhas - that of Understanding and that of Compassion, united as a father and mother. These lights, these initiatory energies, engendered Tara who was then born from the heart of the Lord of the World 'as a bud from the lotus.'
That is how Tara is understood to have come to us -- out of Emptiness, but by the merit of her devotion and her determination which, manifesting as care, finds its way through the union of wisdom and compassion to all sentient beings.