The poem tells the story of the speaker's debate regarding a decision that needed to be made.
There were two possible roads that the speaker could use to travel through the "yellow wood." The speaker needed to determine which road to use. The only difference s/he can see between the two routes is that one of the roads appears to be less frequently used than the other.
it was grassy and wanted wear,
Though as for that the passing there
Had worn them really about the same.
The speaker recognized, with some regret, that s/he probably would not be able to return to travel the road s/he did not choose. S/he decided to use "the road less travelled by" and realized that the decision would make "all the difference" in the way the rest of his/her life took shape.