This, O Nakiketa, is thy fire which leads to heaven, and which thou hast chosen as thy second boon. That fire all men will proclaim. Choose now, O Nakiketa, thy third boon."

In plain words: Yama tells Nachiketas that this heaven-leading fire is now his, as his second wish. People everywhere will call it by his name. Then Yama asks him to choose his third wish.