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.