The two things being compared in "The Road Not Taken" by Robert Frost are the two roads that diverge in the wood. The narrator is walking along a path and comes to a place where the roads forks, at a Y-intersection, into the two roads which "diverge."
The narrator describes the roads as "about the same." As the poem is set in autumn, both roads are covered with freshly fallen leaves. No one has walked on either road recently, since the leaves have not been "trodden black", something that would have been the case had someone walked on them after they had fallen.
When comparing the second to the first road, the narrator describes it as "just as fair", indicating that both roads seem to have equally appealing scenery. Although the road the narrator chooses is grassier, meaning that fewer people have traveled it and a certain amount of grass has sprung up in the dirt or gravel of the road, the poem emphasizes that the roads are otherwise almost identical in appearance.