The poem is built around an extended metaphor comparing life to an old "road." This road isn't always a smooth one: it passes "through pain" and features "many a turn—and thorn." These turns represent the unexpected twists that life can take, while those thorns represent life's sharp stings. This road is also "unfrequented." Few people walk down this path, the poem implies, even though it "stops—at Heaven."
Through this extended metaphor, the poem suggests that a truly righteous life is rare and that the path to Heaven is not an easy one to follow—which, perhaps, is why so many people choose not to bother with it.
The speaker continues to build on this metaphor in the second stanza, following one woman's journey down this difficult road. They point out the various places the woman stopped, as though these were pitstops on a road trip or pilgrimage:
This—was the Town—she passed—
There—where she—rested—last—
The woman wasn't literally trudging through a town or taking naps on the side of the road, of course. These are metaphorical locations, meant to quickly, succinctly tell this woman's life story.
The way the speaker describes the woman's steps reflects the reality of getting older. Her steps were "swift" at first, suggesting the urgency and enthusiasm with which she walked along the road of life. Her "little tracks," or footsteps, were close together.
As time passed, however, her pace slowed. She grew "weary," the poem implies, as life's physical and emotional hurdles began to take their toll. Eventually, she stopped walking altogether, leaving "no other track" for the speaker to follow. In other words, she died, leaving her earthly body—and this road—behind forever.
In the third stanza, the speaker describes the traces of the woman that remain on earth: her "little Book," her "Hat," and her "worn shoe," which "just fits the track" she left behind. The "worn shoe" seems to indicate just how far she traveled to reach her destination, while her book (most likely the Bible) with its dog-eared "leaf—at love—turned back" suggests that she stayed true to her faith in God's abiding love until the end. The mention of these physical objects emphasizes that the woman's soul has disappeared from this world; she doesn't need a hat or shoe anymore, because her "road" has come to an end.