Kaal Sarp Yoga (and Dosha) is an astrological combination in Jyotish. it has importance in mundane as well as astrology.

Kaal Sarp Yoga (and Dosha) is an astrological combination in Jyotish. it has importance in mundane as well as astrology. 

In Hindu tradition, Rahu is a cut-off head of an asura, that swallows the sun causing eclipses. He is depicted in art as a serpent with no body riding a chariot drawn by eight black horses. Rahu is one of the navagrahas (nine planets) in Vedic astrology. The Rahu kala (time of day under the influence of Rahu) is considered inauspicious.

According to legend, during the Samudra manthan, the asura Rahu drank some of the amrita (divine nectar). Sun and moon realized it and they alerted Mohini (the female avatar of Vishnu). Before the nectar could pass his throat, Mohini cut off his head. The head, however, remained immortal due to the effect of amrita and became Rahu. It is believed that this immortal head from time to time swallows the sun, causing eclipses. Then, the sun passes through the opening at the neck, ending the eclipse.
In astrology Ketu represents karmic collections both good and bad, spirituality and supernatural influences. Ketu is associated with the Meena Avatar (Fish Incarnation) of Vishnu. Ketu signifies the spiritual process of the refinement of materialization to spirit and is considered both malefic and benefic, as it causes sorrow and loss, and yet at the same time turns the individual to God. In other words, it causes material loss in order to force a more spiritual outlook in the person. Ketu is a karaka or indicator of intelligence, wisdom, non-attachment, fantasy, penetrating insight, derangement, and psychic abilities. The people who come under the influence of Ketu can achieve great heights, most of them spiritual.