"In the profound sutra system, the Dakini is called the Great Mother.
Indescribable, unimaginable Perfection of Wisdom,Unborn, unobstructed essence of sky,
She is sustained by self-awareness alone:
I bow down before the Great Mother of the Victorious Ones, past, present, and future.
Thus it is written in the Great Paramita Sutra. In the precious tantric tradition, 'desireless, blissful wisdom is the essence of all desirable qualities, unobstuructedly going and coming in endless space'. This wisdom is called 'the Sky Dancer', feminine wisdom, the Dakini.
In the tantra system, the Three Jewels of the sutras are contained in the Three Roots -- Guru, Deva, Dakini. One in essence, these three aspects are the three objects of refuge. Guru is the aspect that bestows blessing; Deva is the aspect that transmits siddhi; and Dakini is the aspect that accomplishes the Buddha's karma."
Trinley Norbu Rinpoche from his Foreward toSky Dancer: The Secret Life and Songs of the Lady Yeshe Tsogyel
translated by Keith Dowman, ARKANA (Penguin Group), London, 1989
See also:
Mother of Knowledge: The Enlightenment of Ye-shes mTsho-rgyal
revealed by Nam-mkha'i snying-po, oral translation by Tarthang Tulku, edited by Jane Wilhelms, Dharma Publishing, Berkeley, 1983