The word itself decode its meaning that is, Pitra – forefathers. Any person whose forefathers have committed any crime, mistakes or sins, then the person is having Pitra Dosh in his horoscope. In simple words, it is paying the Karmic debts of the ancestors.
The Brahma Puran considers the occasion of Shraad very significant. The Puran holds the idea that the Lord of Death, "YamMrityunjay" liberate all the souls on the day of Shraad to go and eat the food made by their children. Those children who do not do Shraad, angry their forefathers returning to their world without eating food. Those children suffer from Pitra Dosh. Shraad comes on the eve of Krishna Paksha of Ashvin month.
Sudden immature death points us to Pitra Dosh. One must consider some examples from our area only. Sudden deaths like "Ishmeet Singh", "Sanjay Gandhi", "Pramod Mahajan", etc.