The poem is spoken by a rock dove, a city pigeon, looking down at people crowding the grey streets. The speaker circles overhead and sees the human world as "all that nothing": a blank, heavy space with no meaning to read in it. Where people might expect signs, songs or rituals, the bird finds none, only the "leaden" street and the crowd moving across it.
The poem stops mid-thought on "but to a child," and that unfinished line is where its feeling lives. We never learn what a child would see, so we have to supply it. The suggestion, judging by the shape of the sentence, is that a child would notice something the adults miss, perhaps the bird itself, or some wonder in the scene. The mood is detached and a little lonely, yet it ends with a small opening of hope. The flat city stays flat, but the broken ending leaves room for fresh, unjaded attention.