The road in Frost's "The Road Not Taken" symbolizes the path of life. At so many different points in our lives, we must make choices. The choices may seem to be small ones, to go down one street as opposed to another or to wear a yellow shirt as opposed to a blue one. But the choices can be large ones, which are more the kinds the poem is intended to make us contemplate. We decide to live in one city or another, we decide to marry one person or another, or we decide to become architects or poets.
All of us would like to perhaps sample these different paths in life, but until we have figured out a way to live alternative lives in alternative universes, we understand that making one choice precludes another nearly all of the time. The narrator says,
Oh, I kept the first for another day!
Yet knowing how way leads on to way,
I doubted if I should ever come back (lines 13-15).
One choice in path leads to another set of choices, and seldom are we able to wend our way back to a previous path-divergence. Even if we do so, we are changed so much by our life experiences that we cannot have a perfect "do-over."
There is some regret and resignation about this, as the narrator says,
I shall be telling this with a sigh
Somewhere ages and ages hence (lines 16-17)
But the fact is, we do have to make these choices on our path through life, and the narrator understands that even though he may have regrets, he must make his choices.