And+be+one+traveler,+long+I+stood+And+looked+down+one+as+far+as+I+could

With this line, Robert Frost is saying that sometimes, you have to do something that you're unsure about, and once you do, you might not be able to go back. I think we can all identify with this point, we've all done things that we wish we could go back and do differently, but the realization is that it's not possible to rewrite some things in your past. Similarly, when at the precipice of your decisions, it's not always clear what lies ahead, but you have to push on anyways.