Next there are verily three worlds, the world of men, the world of the Fathers, the world of the Devas. The world of men can be gained by a son only, not by any other work. By sacrifice the world of the Fathers, by knowledge the world of the Devas is gained. The world of the Devas is the best of worlds, therefore they praise knowledge.

In plain words: There are three worlds: the world of people, the world of the ancestors (the Fathers), and the world of the gods (Devas). You keep a place in the human world through a son, reach the ancestors' world through sacrifice, and reach the gods' world through knowledge. The gods' world is best, so knowledge is praised most.