Three short lines hold a quiet scene: a dark house, a dying fire, and the glow still coming off the coals. The word "afterglow" in the title tells us the main event is over. The fire has burned down, and what remains is the warmth and light that linger.
The surprise is in the last line. The embers "discharge their own great city," as if the glowing coals were a miniature city seen from far above, lit up at night with thousands of tiny points of light. The speaker seems to look closely at the grate and see something huge hidden in something small. "Discharge" is an interesting choice too. It suggests the embers are giving off or releasing this city, almost like breath or a spark of energy.
The mood is hushed, still, and a little wondering. The home is dark and perhaps lonely, but the speaker doesn't sound sad. Instead, the dim light feels full of life, and a small ordinary moment becomes a kind of quiet marvel.