The phrase is part of a change in tone that happens in the last stanza, and suggests (perhaps) the true purpose of the poem. Readers often feel that Frost's point has to do with nonconformity, and the critical choice: choosing the road "less traveled by" somehow "has made all the difference." The poem undercuts this choice however. There really is not very much difference between the roads ("Though as for that the passing there / Had worn them really about the same, / And both that morning equally lay / In leaves no step had trodden black."). The choice of one road over another is not really much of a decision, but a momentary impulse. Nor is it clear what the "difference" might be, or even if it is a difference for the better. In a sense the choice launches the poet on a circular path, which, "ages and ages hence" will come full circle, and leading to a future retelling of this story (perhaps in the form of a poem?). Understood this way, the phrase suggests that the "real" choice has less to do with picking a road and more to do with the stories we tell about such "decisions."

This phrase comes at the beginning of the last stanza of the poem:

I shall be telling this with a sigh

Somewhere ages and ages hence …

The poet has told us that he encountered two roads – a metaphor, for two important decisions to make – and that he chose the less popular one, “the one less traveled by.” Now, in this conclusion, he anticipates a time in the distant future – so distant that it is “ages” away from the present moment – when he will tell others the story of the two roads. Many years will have passed. The poet will have had many more experiences by then. He’s not even sure at this point where he would be living, in this future time: it will just be “somewhere.” Perhaps all of his actions between now and then will have radiated from this one choice he made, which will by then be in the distant past. Even as he makes this decision of which path to follow, he sees this encounter as being one of the most important choices in his lifetime; perhaps, even the most important choice. And this is why he already expects to want to tell people about it, later.