The narrator says in the second stanza of the poem that after looking at the first road, he decides to take the second, as it's "just as fair." By "fair," the narrator could mean "good" and "attractive." In other words, the second path through the woods is just as welcoming and enticing as the first road. However, the narrator could also mean that taking the second road is just as right or as good a choice as taking the first road. In other words, choosing the second path is as right a choice as choosing the first path.
The narrator also chooses the second path because it is "grassy and wanted wear," meaning that the grass is growing long on the second path and it seems as though fewer people have traveled along it. Though the narrator claims he or she wants to return to the first path on another day, the narrator knows that one step often leads to another and that he or she may never return to the first path.