Two short lines carry a surprisingly heavy idea. The speaker starts with something familiar: borders, the lines on maps that separate countries. These, we're told, are "man-made." People drew them, so people can argue over them, move them, or erase them. Then the poem turns. The border "between us" is "natural," and that single word changes everything.
The "us" is never explained, so the poem doesn't say who the two people are or why they're divided. Judging by the contrast, the speaker feels a gap that can't be negotiated away, one that doesn't come from politics or rules but from something deeper, like temperament, feeling, or simple difference. That makes it sadder than any political boundary.
The mood is quiet, cool, and resigned. There's no anger here, just a calm, almost flat statement of fact. The brevity strengthens that feeling: the poem stops abruptly, much like a conversation that has nowhere left to go.