Three short lines hold a whole scene: someone looks, gently, through two sunsets. The poem never says who she is or who is watching, so we're left to piece it together from the images.
The most striking idea is "two sunsets." A sunset is usually something you look at, but here she looks through them, as if they were glass or a veil. One reading is that the sunsets are her eyes, glowing and warm, and the speaker feels the gaze passing through him. Another is that she is looking at something far beyond the ordinary, and the sunsets stand for time, endings, or memory. The poem doesn't settle this, and that openness is part of its charm.
The word "Softly" gets its own line, which slows everything down. The mood is tender, quiet, and a little wistful, since sunsets mean light fading. The speaker seems moved, even awed, by how gentle and deep a single look can be.