The poem begins by introducing an extended metaphor comparing life to an "old—road." This road passes "through pain," meaning that it's filled with hardship and suffering. It's also "unfrequented," or seldom traveled‚ making it a lonely place to be. The road contains many "turn[s]" and "thorn[s]" as well, reflecting the fact that life can be full of—potentially dangerous—surprises.
There's a reward at the end of this long, arduous road, however: it "stops—at Heaven." It is perhaps "unfrequented," then, because few people can manage to walk a righteous path, one dedicated to God.
These lines are packed with caesuras in the form of dashes. Such dashes are very common in Dickinson's poetry, but she takes them to an extreme here. Nearly every line in the poem includes a caesura, and many have more than one! This creates a choppy, halting rhythm that evokes the great difficulty of walking down an obstacle-filled road.
Also somewhat uncommon for Dickinson, this poem doesn't use a very steady meter or rhyme scheme. The lines are generally iambic (meaning their syllables follow an unstressed-stressed pattern: da-DUM), but there are varying numbers of these iambs per life, and the poet often inserts other types of feet.
In the first line for instance, "old—road" is a spondee: it consists of two stressed beats in a row. This makes the phrase sound firmer and denser, again evoking the difficulty of walking this path. Variations like this keep the poem feeling dynamic and surprising—fitting, considering the uncertainty and difficulty of life.
The poem also uses lots of end rhyme, though many of these rhymes are very slant. In this first stanza, "pain," "One," "thorn," and "Heaven" all "rhyme" due to their shared /n/ consonance, but they don't chime together perfectly. Again, this seems to evoke the very nature of the "road" at hand: it's a tricky path, where it's easy to lose one's footing.