The theme is choice; the narrator must make a decision. He can't stand there and vacillate forever, but needs to keep moving forward. He looks at both paths and decides that he will make a decision:
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;
(Frost, "The Road Not Taken," bartleby.com)
Because he is only one man, he can't have the best of both worlds. Choice means exactly that: a choice between two options, without the ability to play both sides against the middle and come out on top. The narrator makes his decision and walks into the future, not knowing how his choice will affect his life.
The setting of the poem is a symbol; the narrator stands at the fork of a path in a yellow wood, looking at two options to choose from in his life. He can't take both options, only one, and so tries to figure out which will be the better choice. Knowing only what he knows at that moment, he make the choice and walks down the "less traveled" path, taking the option that fewer people take in life. The surrounding woods are life itself, continuing and more vast than any one person can understand.
Robert Frost's "The Road Not Taken" seems to intertwine both theme and setting. The poem's opening line establishes the setting as a "yellow wood" and indicates the presence of a traveler who is standing at a place where "Two roads" diverge into these woods.
The remainder of the poem deals with the traveler's observations about each of the various paths and how the traveler reaches the conclusion of which path to take.
Thus, I would say, that in Frost's poem the setting becomes the theme: "Two roads diverged in a yellow wood". Like Frost's traveler, we frequently come to points in our lives where we are faced with two paths from which we must choose (two colleges, two jobs, two cities, two friends, two love interests). Choices like these should bring us back to Frost's poem for further reflection.
thanks alot for useful answer ^^