Polder is a word for land reclaimed from water, and the title sets the scene: a flat, low country where the sea has been pushed back and the ground built up. The poem's few lines turn that idea into a string of odd, soft images. The waters are "deaf as the land," as if both were muffled, unable to hear anything. Then they are "given to wool," which suggests a thick, quiet blanket of fog, cloud or sheep-like softness settling over everything.
The mention of the Dutch fits the title: the place is "overtaken" and "built upon," reshaped by people. The last two lines shift to something playful. A huge rain shower "made a new hat," which could mean the rain made a hat-like cap of cloud or simply that weather keeps remaking the landscape.
The speaker doesn't state a feeling outright. Judging by the imagery, the tone is quiet, dreamy and a little amused. The mood is hushed and gently surreal.