The poem is three short lines, and it works by putting two small things side by side. The title, which reads like a quoted fragment, describes ships seen from far off, so distant they look like flies. The poem itself then compares that sight to the cold leaving the speaker.
The link is distance and shrinking. Ships get smaller the further away they go, and the speaker seems to feel the cold in the same way, as something that was once large and is now slipping off. The poem doesn't say what kind of cold it is. It could be a chill in the body, a cold season ending, or something emotional, like numbness or grief fading. The poem leaves that open.
The mood is quiet and calm, with a touch of wistfulness. There's relief in the cold going, but the word "small" also hints at something being lost as it recedes. The speaker watches without trying to hold on, and the lack of punctuation and the short lines give the poem a gentle, drifting feel.