Offering water to the ancestors.
One stands in one of the sacred rivers and, while reciting mantras, symbolically offers the ancestors the water of the river three times.
It is about feeding the soul of an ancestor because it is believed that there is a time when the gods sleep and the souls receive nothing from the Gods. They come to earth hungry and thirsty and expect food and drink from their families.
Pinda Dana is the offering (dana) of rice balls (pinda).