Why is it that the poem is entitled, "The Road not Taken"? What is the meaning of this poem?

This is Frost in his own words:

"One stanza of 'The Road Not Taken' was written while I was sitting on a sofa in the middle of England: was found three or four years later, and I couldn't bear not to finish it. I wasn't thinking about myself there, but about a friend who had gone off to war, a person who, whichever road he went, would be sorry he didn't go the other. He was hard on himself that way."

Bread Loaf Writers' Conference August 1953

Two roads that were pretty much the same, two paths of life, two choices, presented themselves to the narrator. He chose one of the two paths:

And both that morning equally lay
In leaves no step had trodden black.
Oh, I kept the first for another day!
Yet knowing how way leads on to way,
I doubted if I should ever come back.

That's our lot in this brief existence of ours: we make a few decisions early on, then way leads on to way, and pfft, we're seventy. We know the life we've chosen and lived, but the other life, the road not taken, we'll never have a chance to know where that one may have taken us.