Why is the title of Robert Frost's poem "Mending Wall" ambiguous?

The title of "Mending Wall" is ambiguous because it has two potential meanings. A reader isn't sure if "mending wall" is a noun or an action. "Mending" could be functioning as an adjective that describes wall. For example, a wall could be red, tall, or any other number of descriptors. A mending wall might be a type of wall.  

On the other hand, the title might refer to the action of mending (fixing) a wall. As a reader reads through the poem, the title doesn't become any less ambiguous. Two neighbors do indeed actually mend a broken wall. This suggests the title is meant to be an action. As the poem's speaker contemplates the purpose of the wall, however, his neighbor states the wall is in fact a type of mending wall. It is a mending wall because "good fences make good neighbors." The wall ensures the neighbors maintain friendly attitudes.