The speaker starts with a playful comparison: in strong sunlight, the bare winter woods look like a barcode, the tree trunks standing in neat vertical stripes. But the day described is gray, so that tidy pattern isn't available. Instead the speaker stops beside a diseased chestnut oak and touches its bark.
The surprise comes in the last lines. Up close, the ridges are "kinked and folded," and the lines that are usually straight can't be read at all. The barcode image, which promised the woods could be scanned and understood, breaks down. The sickness of the tree has scrambled the code.
The mood is quiet and a little tender. The speaker doesn't lecture or panic about the tree's illness. They simply slow down, reach out, and notice. There's a gentle sadness in that, along with humility: nature can look orderly from a distance, but close contact shows something more damaged and harder to make sense of.