The word Yantra literally means instrument. A Yantra is a geometric design which acts as a highly efficient focal point for contemplation, concentration and meditation. Yantras carry spiritual significance, and point the user to higher levels of consciousness.
When the mind fosuses on a simple object (e.g. a Yantra), the interference blocking the mental process ceases. Eventually, the object can be dropped when the mind can remain empty and silent without assistance. In the most advanced phases, it is possible to attain union with the Divine by the geometric visualization of a Yantra.
The Yantra is like a microcosmic picture of the macrocosm. It is a focusing point, an outer and inner doorway. In Hinduism, the are many Yantras each of which may focus on a specific deity; so by tuning into the different Yantras you can tap into certain deities or creative force centres in the universe.
A Yantra is usually designed so that the eye is directed into its centre, and very likely it will be symmetrical. Yantras can be drawn on paper, wood, metal, or earth, or they can be three-dimensional.