A man is alone in a forest, and the only company he has is a memory of a woman. The speaker, who seems to be that man, feels caught between the real world and something half-imagined. The "smokey greygreen realities" suggest a blurry place where the trees and his thoughts blend together. Even what he sees seems to come from inside him, as "echoes" travelling from the brainstem to the retina, so the forest may be partly a picture his mind is painting.
The heart of the poem is a promise made to someone who "is / Not". That broken line, with "Not" standing alone, feels like a small drop into emptiness. The person is gone, yet the promise still holds, and he keeps it in his "inner night". The ending is quiet and hard to pin down: twilight "walks back" toward "the shining one", as if the fading light returns to her. The mood is hushed, grieving and loyal, with a faint glow of devotion in the dark.