"Two roads diverged in a wood, and I--
I took the one less traveled by,
And that has made all the difference."
This is probably one of the most often quoted pieces of the Frost poem, but I think it's also best suited for your question. The literal meaning of the passage is that Frost is at a fork in the road. He can go down the path that looks worn and well-used, or he can follow the path that is a bit overgrown and seemingly less used. He knows that the well-worn path probably leads somewhere, but he's not sure where the overgrown path might lead. He's literally deciding which way to go.
Metaphorically, however, the roads represent paths in life. Sometimes we come to a place in life where we need to make a choice. We can't know the eventual outcome, no matter how far down a path we can see, because we can never see to the end unless we venture down that path. I think Frost is implying that this fork in the road is a decision. He can choose the decision that is most often chosen by others, thereby allowing him to see the final outcome somewhat better, or he can make a less popular choice and hope for the best. With the less popular choice, he can't know as much about how things might turn out.
As an example, think of choosing a major in college. You can choose a business major, and you will probably go on to make a nice salary in a corporate environment as many business majors have before you. This is the more trodden road. OR, if you are in love with crafting jewelry, you can take the less popular road and major in crafts or jewelry-making. You don't have as much certainty down that path--you could go on to become the next Tacori, or end up broke and subsisting off of Ramen. You will never know until you venture down the path.