Mending Wall can be appreciated on several levels. The word "mending" can be a verb, making the poem a record of the repair process completed upon the broken stone barrier. "Mending" could also be read as an adjective, a descriptor of a wall that enables the neighbors to maintain a good relationship based on distance.
The speaker in the poem recognizes that sometimes stone walls are damaged due to changes in the seasons and sometimes they are deconstructed by hunters. He realizes that sometimes walls are needed to keep things contained within an area or to prevent intrusion by outside threats. But he questions the need for the wall he and his neighbor are repairing in some areas.
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.
In the end, the neighbor steadfastly holds to his belief that "good fences make good neighbors" and the wall is rebuilt. The reader and the speaker in the poem are left to make their own interpretation of why this is so.