Robert Frost is a poet renowned for his ability to write elegant blank verse. Sound and syllabic stresses are critical components of a poem's musical intonations: how a line may read on the page or out loud. Blank verse consists of unrhymed iambic pentameter, which is a meter consisting of five "iambs." An "iamb" is a type of "foot" (again, a measurement of meter) which consists of an unstressed syllable followed by a stressed syllable. Thus, a line of iambic pentameter consists of ten syllables in a pattern of unstressed, stressed, unstressed, stressed, etc. Let's examine a few lines from "Mending Walls." I will mark the unstressed syllable in italics and the stressed syllable in bold:
And on a day we meet to walk the line
And set the wall between us once again.
In these two lines, there are no errors in meter or other types of feet (like a trochee, which consists of a stressed syllable followed by an unfollowed syllable). Sustaining that kind of meter through an entire poem—not to mention a lifetime of work—is an incredible achievement.
Since all this technical information can get a bit complicated, just remember this: iambic pentameter sounds like the "lub-DUB" of a heartbeat. Try placing your hand over your heart and reading the above lines out loud, and you will find yourself getting a sense of this inherent metering.
Stylistically, this poem is much easier to describe. It is a narrative poem which employs dialogue to characterize the speaker's thoughts, as well as the attitudes of his neighbor.