As the story goes, this represents a great battle in which she destroyed the demon Raktabija. Her black skin represents the womb of the quantum unmanifest from which all of creation arises and into which all of creation will eventually dissolve.
She is depicted as standing on Shiva who lays beneath Her with white skin (in contrast to Her black or sometimes dark blue skin). He has a blissful detached look. Shiva represents pure formless awareness sat-chit-ananda (being-consciousness-bliss) while She represents "form" eternally supported by the substratum of pure awareness.
The demon Raktabija gave the fierce Mother Goddess a fair amount of troubles. He possessed a special magical power which allowed him to create new demons from his own blood. Whenever the Goddess wounded him, each drop of blood that spilled to the ground sprouted another demon full of strength and brutality.
Kali is typically shown as a deranged or wrathful half naked woman, and is often depicted dancing upon the corpse of her consort, the god Siva. She is multi-armed; her tongue protrudes; she wears a garland of skulls, holds a severed head in one hand, and brandishes a hooked blade called a kartri, along with other weapons. Yet to most of her devotees, she is a loving Mother, despite her ferocity.
Kali is one of the many forms of Shakti. Maha Kaali is the fiercest of all goddesses of Hinduism.
The light of Shiva formed her face; Yama gave her hair and Vishnu her arms. From the light of Chandra, the moon God, her two breasts were formed. Indra modeled her waist and Varuna her thighs. Earth gave her hips and Brahma gave her feet.