In Hinduism the yoni is often metaphysically described as the female organ of all creation.
Wikipedia sums up this somewhat ambiguous idea quite well:
Yoni (Sanskrit: योनि yoni) is a Sanskrit word with different meanings, most basically “vagina” or “womb”. Its counterpart is the lingam. It is also the divine passage, or sacred temple (cf. lila). The word can cover a range of extended meanings, including: place of birth, source, origin, spring, fountain, place of rest, repository, receptacle, seat, abode, home, lair, nest, stable.¹
In Hindu temple art female genitalia are often emphasized to symbolize the Great Mother’s crucial metaphysical role in giving birth to all that is.
F. A. Marglin notes that, on a more personal scale, the yoni is said to invigorate men through sexual intercourse.
Popular Hindu Indian folk belief maintains that during intercourse vaginal fluids enter the male generative organ, symbolically known as the linga (roughly parallel to the phallus of the Western mythos). This mingling of bodily fluids is believed to give the male his wife’s spiritual power (shakti).
Accordingly, ancient Kings often had several concubines as their divine right—not only for the gratification of lust but also, so the belief goes, for an increase in spiritual power.²
The yoni and, especially, sexual-erotic scenes appearing on Hindu temple engravings are often interpreted by outsiders as an inferior, crass type of spiritual representation. But Hindus (and Jungians) tend to say that those seeing Indian erotic art and sculpture as “low” or “vulgar” are merely projecting their own unresolved shadow contents.
The yoni is sometimes depicted as a triangle with apex facing downwards. V. K. Chari says that
These geometrical figures have symbolic meanings: the triangle with the apex turned upwards (called vahni kona or cone of fire) may represent male energy, the one with the apex turned downwards female energy (yoni), the matrix of creation, and so forth-which the adept are to meditate upon.³
² F. A. Marglin in The Encyclopedia of Religion. Eliade, Mircea (ed). New York: 1987, Collier Macmillan, Vol. 15, pp. 530-535.
³ V. K. Chari, “Representation in India’s Sacred Images: Objective vs. Metaphysical Reference” in Bulletin of the School of Oriental and African Studies, University of London, Vol. 65, No. 1, 2002: 52-73, pp. 65-66.