Yajna means we have to satisfy the Supreme Person. That is called yajna. And this process can be executed when the human society is very regulated. Regulated means there must be division of these varnas and asramas. Varna means four varnas: brahmana, ksatriya, vaisya, sudra. And four asramas: brahmacari, grhastha, vanaprastha, sannyasa. They have got their respective duties. So unless the human society is divided into these eight scientific divisions and everyone acts according to his position, there cannot be any peace in the world. That is called varnasrama.
Lecture on Srimad-Bhagavatam 3.25.3, Bombay, November 3, 1974