The Shatkona is a symbol for Shiva and Shakti. It is made from two trikonas, Shiva is represented by the upward pointing triangle (△) and Shakti by the downward pointing triangle (▽). Shiva represents the masculine side of God and the parashiva, the all pervasive mysterious form of Shiva without qualities. Shakti represents the feminine side of God and the parashakti, the power of Shiva. The upward-pointing triangle can also represent purusha (the supreme being), and the downward-pointing one Prakṛti, or the world seen as mother nature.
Overlapping they remind us that all these are qualities of one God, neither male nor female but encompassing both (✡). This symbol appears in the twelve-petalled Anahata chakra, or heart chakra. In the West this symbol is more commonly associated with Judaism, where it is known as the Star of David.
Some people speculate that the shatkona was introduced into Israel from India. I have not found any real evidence for this. It could just be one of the common archetypal symbols that arose in different cultures.
The symbol is often found as components in Yantras, or holy symbols. It is also found in Hindu architecture.
An Esoteric Meaning in Smarta Hinduism
The Shatkona has a more esoteric meaning to Smarta Hindus. They see the lower triangle as representing the three states of a human soul Vishwa (waking), Taijasa (dreaming), and Prajna (deep sleep). The upper triangle represents the three cosmic qualities of the macrocosm (God and the Universe); Virat (the physical world), Hiranyagarbha (the subtle world, Eswara, the great Lord, or God.
A mediator can reach a state of Samhadi, where the boundaries between the self and the macrocosm are lost. This state (turia) is represented by the shatkona.
The images in this post are freely available for reuse, and are either self-made or were obtained from Wikipedia.