The poem is symbolically all about choice, and the choice in particular of this speaker to take "the road less travelled by" and the way that this is actually a major decision in his life that impacts the way that the rest of his life will run. The final stanza in its entirety cements the symbolic significance of this poem. Whilst the poet is literally talking about a walk through the woods and the way that he had to choose between two roads, at the same time, the importance that this decision is given, and the way that we are told that he will be retelling this experience "Somewhere ages and ages hence" points towards a deeper symbolic significance. Frost is using the paths in the forest as a kind of symbol for the massive decisions that we must make in life at times.
The final line therefore, in the way that it focuses on the massive "difference" that choosing one path, or one option, over another can make, helps indicate this symbolic significance and illustrates the theme of the poem. Our choices can make "all the difference," and often one choice that may or may not seem very significant can have profound ramifications on our future and where we end up.
The speaker came to a fork and choose the road less traveled. This shows his difference from the others and also his individuality. We can analyze the word difference in two ways. First way is his pride about himself. He choose the road less traveled and something good happened to him and he is proud of himself. The difference is something fine here. On the other hand maybe the speaker is regretful. He use the word "sigh"and also the repetation of "I" shows his regret. If he is regretful this means something harmful happened, and this harmful thing is the "difference". There could be a good or bad thing happened to him but we don't. We can interpretate it in two ways.