In plain words
Besides this, the root cause of this body is the mother and father. Our blood, our food, our brain, our intelligence: all these are the gifts of mother and father. So we owe so very much to the mother and father who blessed us and bestowed on us this sacred human birth. As long as they are alive, we must do the karmas that please them and satisfy them. After they died, these śrāddhas came into being precisely in their memory. It is by such śrāddha rites that we must satisfy those ancestral gods and please them as is fitting. For this reason too Pitṛ-yajña came into being.
Bhūta-yajña. When we bathe, when we sweep the house, when we wash clothes, so many creatures die. To atone for the dying of these creatures, we must perform this kind of Bhūta-yajña, that is, make the right effort to please living beings. It began in ancient times, with the ṛṣis. In the ṛṣis' āśramas, the rearing of deer and peacocks, sāttvika by nature, and the rearing of cows: this was for their own sake and for others', and they loved these hosts of creatures for the joy they had in them. Following our ancient culture, all the people of those days, wherever they saw an anthill, would put sugar there or lay rice flour; all such things they did as karmas to please living beings. After eating, if some grains of rice were left on our leaf-plate, we threw them outside, and some crow or bird or dog would come and eat them; this too they took up as a way of giving satisfaction to living beings. Even today, in many homes, people raise cows, dogs, birds and such small creatures and keep them content. The reason for giving this pleasure to living beings is to atone for the faults we have committed unknowingly.
Next, Mānava-yajña. They call it Manuṣya-yajña. When we light the stove, when we cut vegetables, when we walk along the road, many creatures perish.