From The SpiritWiki
Merkaba (literally chariot of God) is a graphical representation of a fully activated Energy System.
The two interlaced tetrahedrons each represent a portion of the activated energy system. The top pointing tetrahedron represents the three higher chakras (crown, third eye, and voice) while the lower pointing tetrahedron represents the three lower chakras (solar, sacral, and root). The Heart Chakra sits in the middle and balances/helps direct the energies that flow from the upper and lower chakras.