The speaker is describing how they see another person: only that person, and everything else falls away. The way they put it is a little odd. Instead of saying "you fill my view," they say you appear "as a landscape in the evening." The beloved becomes a whole stretch of land, quiet and softened by fading light.
Then comes the strange, precise detail: "one good horse / against the pen." A single animal pressed up to the edge of an enclosure. Judging by the image, it could suggest something strong and valued but also held in, waiting. That may hint at longing, or at a feeling that can't quite get out into the open.
The mood is calm, a little wistful, and very still. The poem is short and spare, and that leaves the feeling unspoken. We sense devotion and tenderness, but also a trace of distance, like looking at something you love across a field at dusk.