Four short lines, and a lot of quiet packed into them. The speaker sits at a window, probably indoors, with a headache, and describes the view as "infinite as a ring." That's a strange comparison: a ring has no end, but it's also small and closed, so the window seems to hold both endless space and a kind of loop the speaker can't get out of. Perhaps the ache in the head makes the day feel that way, vast and circling at once.
The speaker is "sitting backwards," facing away from the view or turned in an awkward position, and so can only listen. What they hear is a dove "speak with the day," as if the bird and the daylight were having a private conversation. The title makes the speaker an eavesdropper, an outsider overhearing it. The mood is gentle, still and a little lonely, but the dove's voice offers some soothing company to someone who is hurting.