Robert Frost's "The Road Not Taken" is frequently taught in high schools as an example of symbolism. A symbol is an image, event, or idea that is meant represent both itself and something else. A symbol is often a concrete (real) object that stands for an abstract idea or concept.
"The Road Not Taken" starts with this line:
Two roads diverged in a yellow wood
These two roads represent a decision that the speaker in the poem is grappling with. We see from the next line that this is not an easily made decision:
And sorry I could not travel both.
He wishes he didn't have to make a decision that eliminated one of the possible experiences offered by this decision.
The two roads are actually similar. At one point he says that the road he chooses is
grassy and wanted wear,
but then he immediately says
though as for that the passing there had left them really about the same.
He also says:
Both that morning equally lay
In leaves no step had trodden black
In other words, they look about the same.
So there is no significant difference between the two roads. We don't really know exactly why the speaker chose the road he did. Sometimes we face decisions like that in life and we just have to pick a direction and start on our way.
To determine the theme we need to consider the end of the poem:
I took the one less traveled by
And that has made all the difference.
A theme is usually defined as a statement about human life that is the central message in a work. A possible theme for "The Road Not Taken" that takes the entire poem into account is:
Difficult decisions may have a significant impact on the rest of our lives.
People must make difficult decisions that will impact the rest of their lives, often without the benefit of much information.