The title suggests riders moving through the dark, and the poem seems to start from the aftermath of something they did. A poor man has taken off his "traveling clothes," as if he had finally stopped moving, and the speaker wonders which tree held his body. Nothing is stated outright. The poem is built from fragments and questions, and judging by the imagery, it hints at a lynching or a similar killing carried out at night.
The speaker never says what they feel, but the feeling comes through in the questioning. They sound shaken, maybe haunted, trying to locate a death they can't quite name. The phrase "so very still" is the quietest and most chilling moment. It describes a hanging body, but it also describes the night itself, as though the whole world is holding its breath. The mood is hushed, eerie, and sorrowful. By saying so little, the poem leaves the reader in the same uneasy silence.