The poem moves through a single day in five short lines of images, and the day seems to shrink as it goes. Morning is "a clear house," open and bright, a place with nothing hidden. By afternoon the house has become "a wardrobe," smaller and darker, a space for storing things away. That change suggests a closing in, as if the day, or a life, is narrowing.
Then comes the odd, quiet turn at supper. The speaker looks over at someone they take to be Susan, but it turns out to be themselves. The poem doesn't explain how this happens, which is part of its eerie effect. It could be a trick of reflection, or a loss of the line between self and other. Judging by the imagery, the speaker may be lonely, or looking for someone who isn't there.
The mood is hushed, a little unsettling, and tinged with sadness. The humor is dry, but the feeling underneath is disorientation and absence.