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"The Road Not Taken" is a poem about making choices and living with the consequences of those choices. That theme can be interpreted on a straightforward level in which the narrator has to make a decision regarding which road to follow while journeying through "a yellow wood." It can also be understood allegorically, as referring to the decisions persons must make regarding courses of action to be followed in life and how those choices impact the remainder of ones existence. A decision is made and a course is selected, with the recognition that the choice could not be revisited later.
I kept the first for another day!
Yet knowing how way leads on to way I doubted if I should ever come back.
Although the choice is made, the narrator still wonders how life would have been different if the other road had been chosen - "I took the one less traveled by, And that has made all the difference."