The Straight Road

[Guinevere]

Well, the closest description of the Straight Road is at the end of the Akallabeth:

Therefore the loremasters of Men said that a Straight Road must still be, for those that were permitted to find it. And they taught that, while the new world fell away, the old road and the path of the memory of the West still went on, as it were a mighty bridge invisible that passed through the air of breath and of flight (which were bent now as the world was bent), and traversed Ilmen which flesh unaided cannot endure, until it came to Tol Eressëa, the Lonely Isle, and maybe even beyond, to Valinor, where the Valar still dwell and watch the unfolding of the story of the world. And tales and rumours arose along the shores of the sea concerning mariners and men forlorn upon the water who, by some fate or grace or favour of the Valar, had entered in upon the Straight Way and seen the face of the world sink below them, and so had come to the lamplit quays of Avallónë, or verily to the last beaches on the margin of Aman, and there had looked upon the White Mountain, dreadful and beautiful, before they died.

But, as Ibrin mentions, these tales are told by humans who "never made the trip".
I think it's meant to be a mystery, intriguing and fascinating just because it can't be exactly explained.