Rahu and Ketu are known as the shadowy planets and they are considered to be the parts of one body, so their movements are interrelated. Actually they are the points of intersection of the paths of the Sun and the Moon when they travel in their celestial spheres. Pandith Vishnu is able to detect the debt of your karma which you might be carrying from your past life. They do so by demarcating the result of their axial movement.
Rahu is the north lunar node and Ketu is the south lunar node i.e. at all times they are just opposed to each other. They carry the secrets of karmic sheaths in a horoscope thus points towards the good and bad things present through generations. In Hindu traditions, Rahu is considered as the severed head of a demon that swallows the sun causing eclipses i.e. considered as a painful condition for the Sun God. Thus these situations are said to have bad effects on our lives. But these are not the permanent situations; if you reach any well-versed astrologer he can give you a feasible solution very easily. It is sometimes that you are told that that your kundli is affected by the Rahu Ketu Dosh. This may cause sufferings in your life. But the suffering without understanding it in this life is a heap shoddier than suffering when you have an idea at least that what it's all for. Now if you are aware that your life is affected by the "kala sarpa" or "kalamrit" or "sarpa" dosha or the planets in your chart are enclosed between those two lunar nodes. You need not worry as Pandith Ji does Puja for the Shanthi or Nivaran. Thus these upshots will bring in peace and glee in your life.