Three short lines hold a small contrast. The title, "Busy," and the phrase "in a mad heat" suggest someone caught up in a whirl of activity, working feverishly through the whole morning. Nothing in the poem says what the task is, which makes the busyness feel general, like the restless rush of any day that gets away from us.
Then the last line shifts the view outward: "mist on the fields." The frantic pace gives way to something soft, pale and still. The mist doesn't hurry. It simply sits there, and the speaker seems to notice it only in passing, or perhaps only at the end of the morning.
The mood is a mix of urgency and calm, and the poem leaves us to feel the gap between them. The speaker may be so absorbed in work that the landscape barely registers, or the mist may be a quiet reminder of what busyness crowds out. The poem doesn't tell us which, and that open space is part of its charm.