The main idea behind a lot of Frost's poem "Mending Wall" is that Frost really doesn't see the use of fences between neighbors. He feels that if they are neighbors, friends, and kind to one another, what need do we have for a wall between each other? It's not like his apple trees are going to cross over and eat his neighbor's pine cones. But, as he repairs the wall with his neighbor every spring, they fix all of the holes and broken parts, keeping that wall erect and standing, even though they are friends. Frost feels it is almost barbaric and savage to insist on a wall between them. At the end of the poem, he sees his neighbor bringing stones to the wall to mend it, and describes him as "an old stone-savage armed". He compares his neighbor to a savage that is armed with a stone, with dangerous intentions. He goes on to say in lines 42-43 that
"he moves in darkness as it seems to me~/Not of woods only and the shade of trees."
These lines are symbolic, because Frost is not saying that he is literally walking in a patch of darkness caused by the woods and shade; the darkness is a metaphor representing the more barbaric, savage part of our human natures that would make a wall between neighbors necessary. He sees for a moment, a time when the world was violent and unstable, and when savages fought each other with stones. He saw back to a darker time when erecting walls was completely necessary to defend your house and home. The darker part of human nature made walls necessary. So, the darkness isn't from the woods or forest, the darkness is a metaphor of darker times when walls were definitely necessary because of the dangerous world that people lived in.