Three short lines hold two very different scales at once. The speaker looks at a window and notices a fly resting on the glass. Its wings are so thin and clear that, when the speaker looks through them, the veins and shimmer seem to turn into snowy mountain fields far away. A tiny, ordinary insect becomes a lens for something huge and distant.
The speaker doesn't tell us how to feel, but the attention in the poem says a lot. There is quiet delight in seeing something unexpected in a plain, even annoying, spot like a dirty windowpane. The mood is still, clear, and a little cool, like the snow itself. The poem also plays with layers: window, wing, mountains. Each one is a kind of filter on the view. It suggests that wonder doesn't need a grand setting. If you look closely enough at small, overlooked things, they can open onto something enormous.