Three short lines carry a small, odd, delightful scene. The speaker steps out into a misty morning, amazed by it ("agog" suggests wide-eyed wonder), and at the gate meets the rising sun. Then the poem turns: the sun becomes a barber, and the speaker is "trimmed."
The title, "Defogging," tells us what is really happening. As the sun climbs, the mist thins and lifts, and the poem pictures that as a haircut, with light snipping away the fuzzy edges of the morning. The speaker may also be getting trimmed in a looser sense, with a foggy head clearing along with the air. That second reading is an inference, but the title invites it.
The mood is playful, fresh and quietly joyful. Nothing is explained, and the humor comes from how casually the speaker accepts the strange image. It feels like waking up and seeing an ordinary morning as something slightly magical.