This poem presents two view of a wall between two orchards. I lived in New England, and these walls are somtimes low walls that were originally intended to keep cows in their pasture (doesn't take much of a wall to keep them in place), but which continued in place long after they were useful. The wall between the orchards in this poem are useless since we're separating orchards and the trees won't devour each other. But this doesn't mean that walls are useless. They key lines for me have always been these:
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.
Sometimes, as in this poem, walls serve no purpose and are only maintained out of habit and superstition. Sometime, however, there ARE things to protect, whether property, or information, or things about ourselves. Just think before you build a wall ... do I need this? Does it serve a purpose? Will it help relationships? If the answer is yes, then by all means build it.
This, as other Frost poems, is delightful in its ambiguity, and there are certainly several interpretations to its meaning. I will offer mine.
The poem is concerned with the obvious divide between two neighbours. Their farms - both arable - are separated by a wall which is annually attacked by both nature and man, and each year the two neighbors embark on rebuilding this divide.
The purpose of mending the wall seems to have different meanings for the two men. For the narrator, it is an opportunity to socialise with his neigbor, to embark on a task together. For the neighbor, it is a necessary act to maintain the gulf between them. The neighbour retains his 'savage' view that -
“Good fences make good neighbors.”
Whereas the narrator sees the wall itself as unnecessary-
My apple trees will never get across
And eat the cones under his pines.
However, for the time that the two men are 'mending wall' they are unified despite their differences.