The narrator of the poem has reached a fork in the road, either literally or figuratively; he must make a choice as to which road to take, with each having qualities that he deems roughly equal.
Though as for that the passing there
Had worn them really about the same,
And both that morning equally lay
In leaves no step had trodden black.
Despite this, the narrator sees that one path is slightly more overgrown, and decides that this path is a better choice simply because it is the less-common or desirable path; he chooses to not follow the crowd and the normal scheme of things, and move in a direction that fewer other people have seen or experienced.
Two roads diverged in a wood, and I--
I took the one less traveled by,
And that has made all the difference.?
(Frost, "The Road Not Taken," bartlby.com)
The narrator's choice is rooted in non-conformity; he believes that while both paths are equal (in other words, neither choice will be explicitly negative), he will come to a different place in life or a different mindset because he has chosen the less-traveled path. While the masses will usually take the first path, it is well-traveled and so there is little new to experience on it; the second path is, by definition, experienced by fewer people. By putting himself into a position to have a different, more unique perspective (literally and figuratively, "a road less-traveled," the narrator will have a more complex outlook on life afterwards.