Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood
And looked down one as far as I could
To where it bent in the undergrowth;
The two roads symbolize, obviously, the choices that the speaker faces in life. He cannot take both, as much as he would like to, so he spends time in comtemplation and observation. He cannot see far, not far enough to make a confident decision as to the better nature of one over the other. The fact that it is a "yellow wood" perhaps indicates that, as fall is often a symbol of the waning years of one's life, the speaker is past his youth, when he can make a choice with the confidence that it is correctible at a later time. The choice he makes will be permanent, highly impacting the rest of his fast-disappearing days. As one approaches middle age, he comes to grip with the fact that his time for hopes and dreams is past; he must come to grips with the reality created by the choices he has made.
Then took the other, as just as fair,
And having perhaps the better claim,
Because it was grassy and wanted wear;
Though as for that the passing there
Had worn them really about the same,
Here the speaker seems to be contradictory. He has made a choice, but is still unsure about it. It is "just as fair" yet it has "the better claim." Then again, there is no appreciable difference as the "passing there / Had worn them really about the same." He is still trying to convince himself that either choice would have been acceptable (just in case this path proves ill-advised). He cannot quite make up his mind about the wisdom of his decision.
And both that morning equally lay
In leaves no step had trodden black.
Oh, I kept the first for another day!
Yet knowing how way leads on to way,
I doubted if I should ever come back.
The speaker is still in the process of convincing himself, even to the point of self-delusion. He tries to tell himself that, should this road proves not the right one, he will have the chance to go back to take the other road. Yet, in a road of complete honesty, he knows that life will probably not allow him the choice to return, even if he should wish to. He has transitioned to the point where he realizes that his youth is past and he must take up the responsility and reality of adulthood.
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 has come to the decision that, for good or ill, the choice he has made will be permanent and highly effecting of his life. He looks ahead to to time when he can look back and tell that the choice he made, whether wisely or unwisely, was the point at which his life's path was set.