Its about the struggle when making decisions in life. There are two paths to follow, the one everyone else does, or the choice that is opposite of the crowd. The person in the poem has to decide to follow the crowd, or to take the other road and make his own path in life, being his own person and not doing things just because everyone else does them. In the end, he chooses to take the path that hasn't had many travelers, and knows that he will be better for it, but also acknowledges that he can never know what would have happened if he had chosen the other path instead.
Here robert frost has given the difficulties faced by him by chosing a road which was not used to much and then he seeies in his past and feels gulty that he did not chose other road and that made the difference