In "Mending Wall," the narrator points out that annually, he and his neighbor rebuild a wall between them that doesn't need to be there. Neither of them keeps animals that might break through; he has apple orchards and his neighbor keeps pine trees, and there's no chance that there will be trouble between them if the wall isn't kept up. Besides, it's a hindrance to the hunters who keep knocking it down (which makes them rebuild it each year). It seems to the narrator that the exercise is pointless.
When his neighbor says, "Good fences make good neighbors," the narrator asks, "But why do they make good neighbors." He makes his case about how the wall does nothing of value, and adds, "Something there is that doesn't love a wall / That wants it down," meaning that they are fighting the forces of nature, essentially--not just the hunters, but gravity and age and erosion.
Toward the end of the poem, he watches his neighbor swinging a stone back to the fence, "like a stone-age savage armed," suggesting that the neighbor is, in some way, like a savage, unintelligent, unlettered, living in the simplicity of life as it comes to him, without questions.
This is when he says, "He moves in darkness as it seems to me, / Not of woods only and the shade of trees. / He will not go behind his father's saying...." This emphasizes the man's simple nature. He lives in the darkness of the trees, certainly, but his darkness resists questions, thought, or change. His darkness is mental--not evil, but unenlightened, simple.