"Fire enters into the houses, when a Brâhmana enters as a guest. That fire is quenched by this peace-offering; — bring water, O Vaivasvata!

In plain words: Nachiketas arrives at Death's house and waits there. Voices remind Yama (here called Vaivasvata, son of the Sun) that a holy guest is like a fire in the home and must be welcomed with water and kindness.