The poem opens with a public hanging at midday, watched by "many fine people." That phrase is dry and a little bitter. The respectable crowd has turned up to enjoy something brutal, and the speaker seems to notice the gap between how these people look and what they are doing. The title, "Darkness at noon," fits: it is bright daylight, yet something moral has gone dark.
The second stanza turns inward. The comings and goings of the crowd leave the speaker "unable to read myself." Judging by the wording, the scene unsettles their sense of who they are, as if being near such an event, even as a bystander, makes self-understanding impossible. The last two lines are the most puzzling: "two lines together / grow a night." Perhaps the lines are the rope, or lines of people, or even lines of the poem itself. However we read them, putting things side by side creates darkness. The mood is cold, uneasy, and quietly accusing.