It's a statement about memory, and the narrative we weave about our lives. The full stanza for the last line:

  I shall be telling this with a sigh
  Somewhere ages and ages hence:
  Two roads diverged in a wood, and I—
  I took the one less traveled by,
  And that has made all the difference.

He is imagining that he will say the last sentence in the future, "Two roads diverged in a wood, and I- I took the one less traveled by, and that has made all the difference."

He is imagining this despite the first three stanzas being about how similar the two roads are, how equally traveled, and how he regrets that he'll only be able to take one. He knows he's making the choice at random, and knows that in the future he will remember otherwise.