What is kala sarpa dosha?

What is Kal sarapa dosha?
Kala sarpa dosha is caused by the Shadow planets rahu and kethu. When all the seven other planets sun, moon, mercury, venus, mars, jupiter and shani are hemmed beween the rahu-ketu kal sarpa dosha is formed.
For example in your chart Rahu is posited at simma rasi (leo) and ketu at kumbham (Aquarius) rashi. If all other planets are placed at meenam, Mesham, Rishabam, Mithunam and Kadaka rasi this dosha is formed. If any one planet falls out side there is no dosha.
Why kala sarpa dosha is feared?
This dosha is believed to weaken the power of other planets and render them useless. So a person with this dosha suffers during his early life. This dosha will weaken in the later years of the life.

This dosha is different than the naga dosha which exists in birth horoscope. Naga dosha is not kala sarpa dosha.
Late prime minister of India pandit Nehru had kala sarapa dosha. It is true that he suffered a lot and spent many years in jail for India's Independance. Later he became the prime minister of India, a great privilege and supreme post.

What is the parihara (remedy) for kala sarpa dosha?
There exist no parihara according to ancient jyotish classics. If your jathaka has kala sarpa dosha even if other planets are powerful and it kendra sthanas, you will not enjoy the benefits of these grahas. In practical I have this dosha in my horoscope. I have four planets in ucha (exalt) houses. Still this rahu dasha is proving very tough for me. I suffered a lot in profession and earnings.
Thirunageswaram is the sthala temple for rahu. You can go there and do poojas for rahu. During the rahu dasha you can chant manthras for rahu and wear snake shaped rings specifically designed with gems for rahu and kethu.