The narrator knows that he must make a choice, regardless of the outcome. He cannot take both paths, and ultimately decides to take the one that he thinks is "less traveled" because of the overgrown grass. This symbolizes a person's unusual choice, not one that is commonly made by most other people. As he walks, the narrator thinks that might do two things in the future:
Oh, I kept the first for another day!
Yet knowing how way leads on to way,
I doubted if I should ever come back.
I shall be telling this with a sigh
Somewhere ages and ages hence.
(Frost, "The Road Not Taken," bartleby.com)
His first wish is that although he has made his choice, he might be offered the opportunity to experiences the second someday. However, he knows that this is unlikely; having made his choice, he will almost certainly never return to the same circumstances that led to his first choice.
The second, more of an assumption about his future, is that he will be able to look back on this choice and second-guess it "with a sigh," since he might live to regret his choice. However it turns out, he will always wonder about the other path, and wonder if his choice was the right one; his memories might make the other path seem more alluring as time goes by, but he knows that his choice is definitive, and he must continue on.