Pitra dosh is a term related to astrology. A specific conjunction in the natal chart is termed as pitra dosh.
Generally if the ninth house (the house of fathers) of the natal
chart contains the malefics sun and rahu, then it creates pitra dosh.
Yes, it is said that this conjunction arises in the chart if the pitaras (deceased forefathers) are unhappy with the person. He might have neglected doing shardaha for them and may have disrespected them for which his ancestors got unhappy. Just like us pitaras require satification through food and respect. This is done through the shradha ceremony. So it is said that when a person had neglected such duties, in his chart pitra dosh arises.
Pitra dosh in the birth chart creates unhappiness in life, causes failure in personal endeavors like job, business, etc. To get rid of pitra dosh astrologers suggest performance of different kinds of pujas, shradhas, tarpanas etc. If you are really interested to know, you should better consult a good astrologer.
Disclaimer: I have provided the answer only as per my awareness about astrology, I am not an expert.