How does an understanding of "mending" in "Mending Wall" as either verb or an adjective affect the interpretation?

This question points out how the word "mending" in the title of the poem can be either a verb (something you do to a wall) or an adjective (something that describes or defines the wall).

By noticing that you can interpret that word both ways, you understand that the title is purposefully ambiguous: that it provides us with two ways of thinking about the poem. In other words, to answer your question, an understanding of both grammatical ways to interpret the word "mending" in the title helps us realize that the poem is not just about a single activity but that it can mean more than one thing, and so can the objects or symbols within the poem. This matters because it helps us realize that we shouldn't put the poem aside after understanding its literal plot. We should continue looking for multiple meanings.

The first way to understand the poem's title is as a verb and its object: "Mending Wall," like in other "verb + object" phrases like "fixing dinner" or "doing homework." On this level, we understand that the two neighbors in the poem are literally mending (repairing) a wall together.

The second way to interpret the title is as an adjective and it noun it describes: "Mending Wall," like in other "adjective + noun" phrases like "falling rain" or "running squirrel." On this level, we also understand that, first, the wall itself is something that is capable of mending--that it provides a way of fixing up the relationship between the two neighbors--and second, that the wall's purpose is to be mended, again so that it can bring the neighbors together and give them something meaningful to do while they spend time in each other's company.

Knowing that we can interpret the title in multiple ways helps us consider multiple meanings for the wall (as a means of both separation and connection, for instance) and multiple meanings for the dialogue spoken in the poem (for example, how "Good fences make good neighbours" can be an indication of fences' ability to set boundaries as well as allow neighbors to meet and connect).

You can usually assume that titles are chosen carefully by authors. Whenever you think, "Wait, but this title could mean two things!" then the author probably meant for you to notice that and to accept both interpretations of the title as equally valid.