A yantra is a mystical diagram that is the house in which a Devata (God or Goddess) dwells. Worship of planetary yantras is used to remove the ill effects of malefic planets and to increase the positive influences of favorable planets.
Ketu is not a planet, but the descending / south lunar node. Ketu is considered as the Tail of the Demon Snake and is generally referred to as a "shadow" planet. Ketu is one of the nine primary celestial beings in Hindu Vedic astrology who are the agents of the Law of Karma.
The yantra shows the south lunar node symbol and the mantras associated with the ruling deity, Matsya Deva. The two lunar nodes are generally considered malefic for they indicate factors that cause eclipses that block out the light of the Sun and the Moon. In the spiritual realm, Ketu gives perception, liberation, wisdom and psychic sensitivity.