Yama said: "I tell it thee, learn it from me, and when thou understandest that fire-sacrifice which leads to heaven, know, O Nakiketas, that it is the attainment of the endless worlds, and their firm support, hidden in darkness."

In plain words: Yama agrees to teach the fire ritual. He tells Nachiketas that understanding it leads to endless worlds. It is their firm foundation, though it is hidden and hard to see.