Robert Frost, in his poem "The Road Not Taken," reflects on choices in life, and specifically the fact that some choices we make change our lives and our selves in dramatic and irreversible ways. The alternate theme is that had we taken the other road, made the other choice, we and our lives would have turned out very differently.
Taking the poem literally, we find a man out for a walk in the woods. He comes upon a fork in the road, where two roads lead off into different directions. There is little difference between the roads in terms of appearance, although the one he decides to take appears to be slightly less traveled "Because it was grassy and wanted wear" and for that reason it has "perhaps the better claim" on him (Frost lines 7-8).
After saying this, however, the speaker adds again that there is not really a great deal of difference between them and that neither is well traveled. He tells himself, with some excitement (signified by the "Oh" and the exclamation point), that he will return another day and go down the road he has rejected.
Then he retreats from the excitement and the pledge to himself to return to go down the other road, and instead he realistically concludes that he will probably never come back and go down that first road, that his choice to go down the second road will lead on to other choices and other choices and on into the future, leaving the first road---like all other rejected choices---way in the past where it will remain.
Finally, the narrator reflects on his future, with respect to the decision he has made to go down the second road and reject the first. He thinks about how, some time in the distant future, he will be telling the story of choosing the road "less traveled by" (ln. 19), and how "that has made all the difference" (ln. 20).
The final lines mean that the speaker will some day be at a point when it has become clear how that choice in roads has changed his life and has made...