The road the speaker is on comes to an intersection in a wood. He has a choice of two roads to continue on. One of them goes off until it bends and is lost from sight in the undergrowth.
The author takes the road that, he says, was less traveled by. But the thing is that there is really not any difference between the two roads. The author says that one is grassier and less trodden down, but then says the other is just the same. He says both roads had equally untrodden leaves.
So the difference is all in the speaker's mind. He says he will look back and say that he took the less traveled road and that that made all the difference. But we know both roads are really the same as far as he can tell right now.