As another reviewer said, you could look at "mending" either as an action being performed by the speaker and the neighbor or as modifying the type of wall they are working on together. The title is also ambiguous because the wall itself is an ambiguous symbol, and the speaker has mixed feelings toward it.
The speaker begins by describing how he and his neighbor, whose properties are separated by the wall, get together each spring to mend the wall after the damage it suffers each winter. The speaker is immediately hesitant, though, as he begins, "Something there is that doesn't love a wall" (line 1). He goes on to explain that "the frozen ground-swell," "hunters," and "the rabbit" all work to topple the wall. This is why the wall needs to be repaired each spring. The speaker begins to wonder—if all of nature is working against the wall, then why should humans keep up this useless process of repair?
The answer, of course, is that the neighbor believes in the necessity of the wall. He is the one who repeats the famous adage "Good fences make good neighbors,'" while the speaker wonders why. The ambiguity of the title could reflect the speaker's ambivalence toward the wall. On the one hand, he wants to keep peace with his neighbor; but on the other hand, he feels offended that the neighbor feels the need to "wall [him] out." While the neighbors are "mending a wall," they may actually be dismantling their relationship or the civility between them. In this sense, the title can also be considered ironic.