Ten words are all the poem has, and the title carries almost as much weight as the lines. "Time to retire" can be said by someone closing a book at midnight, or by someone looking back over a whole working life. The poem never picks between these meanings. A moth lands on a hand, and the speaker, who seems to be awake and busy long after he should not be, sees something like himself in it.
The first couplet is all surprise and touch. "A sudden moth" interrupts, and "on my hand" brings it down to the scale of skin. Moths are nearly weightless, but the line break after "moth" gives it a small moment of arrival before the hand is named. Judging by the poem, the speaker is still, perhaps working at a desk or reading, since a hand that stays put long enough to be landed on is a hand that has stopped moving. The poem does not mention a lamp, but a moth out at night suggests one, and a light burning late is the quiet setting under every line.
The second couplet turns the encounter into a question, or something like one. "Who works so gray / so late" has no punctuation, so the reader cannot tell whether it is an observation, a rhetorical question, or a kind of tired sigh. The word "who" matters. The speaker could have written "that," as English usually does for insects, but "who" gives the moth a personality, almost a job title. Then comes "works." A moth's circling around a light looks like labor: restless, repetitive, never quite getting anywhere. The poem invites us to see the moth as a colleague putting in overtime.
"So gray" is the oddest phrase, and probably the richest. Gray is a color, but here it is stretched into an adverb describing how the moth works. That makes the color a mood: dusty, dull, worn down. Gray is also the color of twilight, of ash, and of aging hair, so the word quietly brings in the retirement of the title. By the time we reach "so late," lateness is both the hour and a stage of life. The repeated "so" sounds almost like a sigh. It stretches both phrases into an exhausted, sympathetic tone, as if the speaker were saying "look how gray, look how late."
The sound of the poem supports this. The lines run four syllables, three, four, then two, so the poem shrinks as it goes and ends on the short, falling "late." There is no end rhyme, but "gray" and "late" share a long vowel. After the clipped, closed sounds of "moth" and "hand," those open vowels slow the voice down and let it trail off, much like someone running out of energy. The absence of capitals and punctuation adds to the hush. Nothing is announced, and the poem seems to be spoken quietly so as not to startle the moth.
The poem's meaning depends on the reader. The title can be heard as the speaker's own thought, or as a message delivered by the moth, a small gray visitor who seems to say that it is time to stop. The moth can be a reproach, since it works as hard as the speaker does, or a companion, since the speaker is no longer alone at that hour. In both readings the speaker has recognized himself in a creature that cannot recognize him back. That gap gives the poem its gentle pathos. The moth does not know it is being compared to anyone. It is simply there, gray and busy, and the speaker projects onto it his own tiredness.
That is why such a small poem is worth reading closely. It shows how much a short text can hold if every word does more than one job: "retire" as bedtime and as the end of a career, "gray" as color and as age, "late" as hour and as life stage, "works" as the moth's flutter and as the speaker's labor. The poem makes no argument about work or aging. It lets a moth land on a hand and leaves us holding the question of who is working so late.