In The Road Not Taken, the speaker has come to a point in his life where he can no longer look down " as far as I could" and he has to make some kind of potentially life-changing decision. He has a choice - one OR the other- as he cannot "travel both."
The fork in the road then represents a stage in his life and even though he has tried to envision what the road he does NOT take will be like by considering it up "To where it bent in the undergrowth," he decides to take "the other."
The speaker's confusion is apparent as he considers the less popular choice because "it was grassy and wanted wear" but on reflection decides that perhaps both paths that meet at this fork have equal merit as they are "worn ...really about the same." The fork thus also represents his indecision.
Equally the fork represents seriousness of his dilemma and the finality of his decision once taken as "I doubted if I should ever come back." The speaker is hopeful that the choice he has now made is the best one and he shall look back "Somewhere ages and ages hence" and know that having to make this decision, being in this position "has made all the difference.