Maraka planets are ones that get qualities of causing death by virtue of ownership of certain houses in the Vedic horoscope.
In Brihad Parashara Hora Shashtra, Maharshi Parashar has commented that 3rd and 8th house in a horoscope are houses that denote length of life. The vyayasthanas (12th) from these houses are bestowed with the quality of expending away life. Overall longivity should still be seen from 3rd and 8th, but the cause and timing of death should seen from rulers of 2nd and 7th houses. Inimical planets in these houses should also be seen in the same light.
Why suddenly a discussion on Maraka planets? While recognizing and understanding that death is the finality, I am hail and hearty, thanks for asking.
We always associate life and death with an organism, very interestingly this works for various aspects denoted by the horoscope as well. During the recession years of 2001-2003 in the US, my company went under and I too lost my job like lakhs of other engineers. I was running mahadasha/antardasha of rulers of 4th and 11th house – 7th and 2nd from 10th, the house that denotes profession and career.