The speaker is hungry, and the house is empty of food. Instead of real dinner, they "bite into" a kind word, treating it like something edible. That small, odd image carries the whole poem: kindness becomes nourishment, and its absence at the table makes it feel all the more precious.
The last line is the strangest and most striking. "The great dark ink of it" suggests the word is rich, heavy and bittersweet, like something that stains the tongue. Ink also points to writing, so the kind word may have come in a letter or message, or the speaker may be turning it into a poem right now. The poem doesn't say, and that openness is part of its charm.
The mood is quiet and tender, with a thread of loneliness underneath. The speaker doesn't complain about having nothing to eat. They simply call the word "a treat," and that plain gratitude makes the poem moving. A little warmth goes a long way when there is little else.