Frost writes this poem to question the validity or usefulness of walls.  He questions whether they do more harm than good; after all, if you erect a wall between you and your neighbor, isn't that a bit insulting?  It's like saying, "Hey, we're neighbors and all, but I sure don't trust you...you might come over and steal my stuff, so, this wall will sure keep you out."  He questions the phrase, "Good fences make good neighbors" that his neighber keeps quoting at him.  He thinks to himself,

"Before I built a wall I'd ask to know/What I was walling in or walling out, /And to whom I was like to give offence"

by building the wall.  His neighbor has pine trees and he has apple trees, and it's not like his apples will cross the border "and eat the cones under his pines," so really, why have a wall?

So, Frost teams up with his neighbor to repair holes in the wall, and says in lines 14-15 that they

"set the wall between us once again. /We keep the wall between us as we go."

The repetition enhances the feeling of separation that he has with his neighbor.  They don't both stand on the same side of the wall as they repair it, they keep the wall between them.  Even though the seasons and hunters have torn holes in the wall, they work hard to keep that wall preserved. Frost uses the repetition to contrast two friendly neighbors who are out working together, but still have to have a wall there.  He thinks it's absurd, and the repetition of "the wall" emphasizes the absurdity of it.  The wall transforms a friendly neighbor into "an old stone-savage armed" with rocks, coming from a darker and more dangerous time.

It's a great poem the provides a lot of thought-provoking points about human nature, tradition, and logic.  Good luck!