First, we're presented with an unusual image—a house that's been far out at sea, all night. Was it literally out at sea? Is it still out at sea? We're not allowed to know—not yet, anyway.
Regardless of what is later revealed, this is an eerie image of isolation.
If you imagine a house floating alone on a sea, it emphasizes the smallness and fragility of the house and of the people in it. The house is surrounded by a powerful and mysterious force—the ocean—which is capable of destroying it.
The woods crashing through darkness, the booming hills, Winds stampeding the fields under the window Floundering black astride and blinding wet
At this point, we start to figure out what's really going on. We're definitely not out at sea anymore—we're on land. So maybe the house was on land all along, too?
The woods crash through darkness, and the hills boom because… the wind is blowing really heavily. (Ah, so now the title of the poem's starting to make sense.)
The wind is like some sort of animal. In the plural, as "winds," these wind animals are stampeding fields, "floundering" in an ungainly way astride the blackened world in the blindingly wet weather. The wind is personified as a creature, one that seems to be almost panicked and running over the earth.
Note the brief instant of assonance here, between the long I sounds in "astride" and "blinding." Check out "Sound Check" for more on this poem's sounds.
At this point, looking back at the first line, it starts to seem like the house wasn't literally at sea all night. It was just isolated in the middle of a windy landscape, which—come to think of it—is kind of like being at sea.