Four short lines, and almost no punctuation or explanation: the poem hands you a few sharp images and leaves you to connect them. A scrub fire is a quick, rough blaze that races through dry brush. Here it seems to burn "against the purpose," as if something is working in opposition to meaning or intention. Then the fire is "eating everyday experience," so the thing being consumed is ordinary life itself.
The poem doesn't say how the speaker feels, and that coolness is part of its effect. It reads like a note jotted down while watching something spread. You could take the fire literally, as a landscape burning, or as a picture of how distraction, anxiety, or routine can wear away our sense of why we do things. The title, "Antithesis 6," hints at opposition and at being one piece of a larger series.
The mood is tense and unsettled, yet oddly still. Nothing is resolved, and the last word leaves the burning unfinished.