Literally, it refers to the road that the narrator took, grown over and unused. The concept is that the unused road might be more interesting than one that everybody uses.
More abstractly, it refers to the path (in life) not taken, or the choices one makes. British author Terry Pratchett uses a concept in his stories called the Trousers of Time: a fork in your life where your choices diverge; take one, and your life is irrevocably changed.
...[I] looked down one as far as I could
To where it bent in the undergrowth...
This could mean that, confronted with a choice, he considered one as much as possible before taking the other; one choice may have been safer, but he took the "one less traveled by," or the choice that others may not have taken.