The poem imagines two people who haven't met yet, a "she" and a "he," each living a complete life but quietly feeling something is missing. The speaker describes them in pairs of opposites: strong and vulnerable, simple and sophisticated, everything and nothing. That makes them feel like ordinary people and also like figures in a fairy tale.
The middle of the poem is wistful. They live, eat and sleep, yet they dream of someone else, and the images of a lost puzzle piece and a missing chord show that each sees the other as the thing that would finish them. The grand gestures, climbing to the moon or asking God for a heart, add a touch of romantic exaggeration.
The mood is hopeful and gentle, with a little loneliness underneath. The ending is the neatest part: after all the separate "she" and "he," the final line gives us one word, "Us," standing alone, as if the poem itself were the moment they finally come together.