The speaker is looking back on a final parting. One windy afternoon they spoke a last word to someone, and they never met again. Now they wonder where that "unknown friend" has gone. The word "unknown" suggests the person may be a stranger in some ways, or simply lost to the speaker's knowledge.
The feeling is a quiet, tender longing. The speaker has memorized the other person's face, even the small moment when a thought was held back instead of spoken. Walking on thin grass and talking to themselves, they say they are really talking to the absent friend. Laughter and pity for the crows drift through these memories, giving them a gentle, dreamlike softness.
Near the end, the speaker imagines the friend entering darkness and becoming something divine, "clothed in white wind." The poem closes with the speaker lying among restless leaves, offering a last thought. The mood is wistful, peaceful, and a little haunted, as if grief had turned into a kind of prayer.