There are two basic types of walls between the two main characters in Mending Wall. One is physical, the other is mental.
The physical wall is the rock wall along the border line between the speaker's apple orchard and the neighbor's pine trees. This wall is created as much to remove the rocks and boulders from the ground where trees and grass and other crops might be trying to grow as it is to mark the dividing line between properties.
The mental wall is the separation preferred by the neighbor as he insists, "Good fences make good neighbors." The speaker in the poem is not convinced of this and wonders,
Why do they make good neighbors?...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.
The neighbor, however, is steadfast in the belief first expressed by his father, and so the two continue to repair the wall for another spring.
The poet and his neighbour continues an act of reparing a broken wall. Apperently it's thet wall they talk about. Yet there's an underlying meaning.
The poet considers this act as archaic and old-fashioned and want to convince his neighbour about the uselessness of doing it. but his neighbour contradicts repeating an old proverb "Good fences make good neighbours". but this neighbour doesn't seem to understand its real meaning. I think it means we have to have a distance with everybody, because too much of togetherness always lead to quarells. It does not mean we have to create walls, but something that should be created in mind.
Poet wants to convince the fact that his apple trees are never likely to encroach neighbour's pine trees, which later he realizes invain because his neighbour is a fuddy duddy person. His way of thinking is the barrier or the wall disscussed here.