Three short lines, and a small moment of hurt sits inside them. An old friend walks past, and the speaker notes that no greeting came. That's all we're told, and the poem's power comes from how little it says.
The speaker never states a feeling, but the quiet way the moment is reported suggests disappointment, maybe confusion. The dash before "didn't say 'hi'" works like a pause, as if the speaker is still taking it in. The word "old" matters too. This is someone with a shared history, which makes the silence sting more than it would from a stranger. We can't know why the friend didn't speak. They may have been distracted, or avoiding the speaker, or simply didn't notice. The poem leaves that question open.
The mood is subdued and a little wistful. It captures the way a tiny slight can carry a lot of weight, especially when it hints that a friendship has faded.