In plain words
So the great souls of those days, holding yajñas of this kind, regarded as yajñas the karmas done to atone for these creatures that died, to atone for sin.
Last is Ṛṣi-yajña. They call it Brahma-yajña. Jantūnāṃ nara janma durlabham. This human birth is very hard to gain. Durlabho mānava janmaḥ. This birth as a man is very hard to come by. It has come about only through the merit of past births and the grace and gracious glance of God; it was not gained by human effort. To put this human birth, once gained, to use in the proper way, to set it on the right path and bring it to its goal, the ṛṣis of those days laid down many śāstras, Upaniṣads, itihāsas, rules and restrictions, and showed man a way. It was they, too, who laid down the four puruṣārthas, Dharma, Artha, Kāma and Mokṣa. They set out these ideals to lead man's daily life onto the path of Satya. For man they laid down restrictions that no beast, bird or animal has. The reason is this: since man has the knowledge to discriminate, since he has the power to examine the eternal and the transient, he must keep to such a way. These sins too exist only for man; the other beasts, birds and animals have none at all. They laid down commandments too only for men; for beasts, birds and animals they did not lay them down. The śāstras too came into being for men, not for beasts, birds and animals. Curses too came into being for men, not for beasts, birds and animals. Sin, śāstra, commandment and curse: all four belong to man alone. So the maharṣis set forth many kinds of ways, bidding man walk this sacred royal road and make his human birth holy. For this way they have given, our chief duty is to think of them gratefully, to remember them, to meditate on them, and to fulfil their ideals.
Along with this, man ploughs the land to grow crops.