Yantra (यन्त्र) is the Sanskrit word for "instrument of liberation". It is a mystical or astronomical diagram, usually a symbol, often inscribed on an amulet, and said to possess mystical or magical powers.
The act of wearing, depicting, enacting and/or concentrating on a Yantra is held to have spiritual or magical benefits in the Tantric traditions of the Indian religions. In some disciplines of Tantra it is said that a focused, controlled gaze upon a particular yantra may lead to liberation.
Kali is the Goddess of time, and meditating on the Kali Yantra is said to speed up your spiritual evolution.