The main idea of this poem is that the choices we make will change our lives forever.
The narrator, in the third stanza, says "Oh, I kept the first for another day! / Yet knowing how way leads on to way, / I doubted if I should ever come back." He meant to try the other path (the other choice) later on, but because life takes us through so many twists and turns he doubted that he would ever actually get to see what that choice held for him.
In the final stanza he states that taking the less traveled road "has made all the difference." His one choice changed his life forever, and because of the word "sigh" in that last stanza we can assume that maybe it was not changed in a good way.
