Short as it is, the poem hangs on a strange idea: that fire is a god, and that anyone who speaks to it must become its servant. The opening line, "speak fire to fire," suggests answering flame in its own language, not putting it out or ordering it around. Then comes the odd demand that "it must have many hands." Fire seems to need endless helpers, or perhaps the speaker is imagining flames themselves as hands, reaching and grabbing in every direction.
The comparison to "a housekeeper / with one angry god" gives the poem its tension. A housekeeper does small, practical, endless work, yet here she answers to something volatile and impossible to satisfy. The speaker seems to feel both awe and weary resignation, the way you might feel tending a stove or a temper. The mood is hushed and slightly uneasy: devotion mixed with fear, and a sense that serving something powerful never really ends. The title, "Servant," makes that quiet burden the point.