This is a metaphor for a decision that changes everything – once the person has made it he can never go back.
The last verse begins with a repetition of the first line which brings the reader full circle and back to the extended metaphor. One of the key words here is “sigh.” This is the indication that the speaker’s choice was not as successful as it might have been.
And then the famous line "and that has made all the difference," solidifies the figurative level of this poem by saying that taking the road that the speaker took, making the choice that he made, has changed his life.
The speaker realizes that, despite being faced with two relatively equal options in the moment, he will look back on his decision and assign potentially misguided importance to the fact that he chose one road over the other. In hindsight, he will have chosen “the one less traveled by.” Depending on the “difference” this decision made in the speaker’s life, he will either look back on this decision fondly, patting himself on the beck for making the right decision or he will look back with regret, chiding himself and pining after things that could have