The speaker passes along something someone once told them: we aren't really afraid of darkness itself, but of what it hides. The fear is about not knowing, about the chance that something harmful is nearby and we can't see it coming.
The poem's final line turns this idea toward people. The speaker suggests that other people can be like the dark, concealing what might hurt us. Even more unsettling is the earlier image of danger standing "right before" our eyes and not registering as a threat. That suggests the harm isn't always hidden at all. We may simply fail to recognize it.
The tone is quiet and reflective, with a hint of unease. The speaker doesn't spell out any particular person or event, so the poem judging by its wording feels like a lesson learned the hard way, or at least a warning. The short, unfinished-feeling last line leaves the reader with a chill.