Well done for identifying the deliberate ambiguity of the central image of the wall. Of course, this ambiguity is created in part thanks to the other word of the title, "mending," which is used as both a verb and an adjective in the poem, so we are never too sure how this central image is to be read. However, it is suggested that the act of mending the wall is one that is used to help maintain the relationship between the speaker and his neighbour. Of course, the wall refers to two kinds of barriers that we erect in our lives, both emotional and physical. There is of course the literal wall that the neighbour of the speaker feels is so important to maintaining good relations, and then there is the emotional barrier that is erected between them, which the wall stands as a symbol for. The attitude of the neighbour towards the wall is repeated, parrot-style, again and again in the poem, perhaps reflecting his complete uncritical faith in the value of a wall:
"Good fences make good neighbours."
However, the speaker finds this answer unsatisfying, as the act of mending the wall makes him think philosophically about some of the deeper implications of what he is doing:
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.
Something there is that doesn't love a wall...
Whilst for the neighbour a good barrier ensures a healthy relationship, for the speaker, there is the troubling thought of what offence might be caused by building that wall, given the inevitable way that you exclude and include others by the act of building a wall. However, the fact that the poem ends with the litany of the neighbour actually suggests that it is he who has the more enlightened and sensible view: given human nature, clear boundaries perhaps may be very important.