3.14 All living beings subsist on food, and food is produced by rains. Rains come from the performance of sacrifice, and sacrifice is produced by the performance of prescribed duties.
Here, Lord Krishna is describing the cycle of nature. Rain begets grains. Grains are eaten and transformed into blood. From blood, semen is created. Semen is the seed from which the human body is created. Human beings perform yajñas , and these propitiate the celestial gods, who then cause rains, and so the cycle continues.