It is believed that during Pitru Paksha, our ancestors comes on earth. From Pitraloka, a region between heaven and earth, they visit earth to meet their relatives and bless them. Tarpan, Shradh, and Pindadan are rituals in hinduism to appease the fathers in Pitru paksha. By doing this in Pitru Paksha, one gets freedom from Pitra Dosha too.