Firstly, it's important to understand that "the road" in "The Road Not Taken" is metaphorical—not exactly a literal road, though the poem may have been inspired by an actual walk through a forest. The road that the narrator has taken is "the one less traveled by." The narrator describes it in relation to the first as follows:
And be one traveler, long I stood
And looked down one as far as I could
To where it bent in the undergrowth;
Then took the other, as just as fair,
And having perhaps the better claim,
Because it was grassy and wanted wear;
Though as for that the passing there
Had worn them really about the same,
And both that morning equally lay
In leaves no step had trodden black.
Oh, I kept the first for another day!
The first path seems attractive, due to how it bends "in the undergrowth," making it shadier and more mysterious. However, the narrator takes the other, which is still "grassy and wanted wear," indicating that few have trodden on this path. This description of the second road is the reason why many people read the Frost poem as championing non-conformity. I would say that they are not exactly wrong. It seems that the narrator chooses the second path because it seems to offer something new. However, on a closer look, he sees that they are worn "about the same" and both are covered in freshly fallen leaves. He imagines that he will have a chance to travel on the first path on another day, but there is never time for that opportunity.
The poem is a meditation on choices and how we cannot enjoy all of the opportunities that are offered to us in life. The choice of one thing guarantees the loss of another.