A person is alone in a kitchen, reheating yet another meagre meal. That small, tired moment opens the poem, and everything after it grows out of it. The speaker suggests that most people never see the "full mask," the whole of what lies behind appearances or behind a life like this one. Some even jeer, treating the matter as a simple tally, "a common count," as if a person could be added up like numbers.
The last two lines turn the poem. Seeing "so little" is called "one of the greatest arts," and the tone is hard to pin down. It could be dry irony, a sour joke about how people get by through ignoring what's hard. It could also be a quiet kind of coping, a way of narrowing your attention so you can keep going. Judging by the thin dinner and the brevity of the lines, the mood is wry, subdued and a little lonely, with an uneasy sense that not seeing is both a skill and a loss.