The Straight Road is the route that leaves the earth's curvature through sky and space to the ethereal land of Aman. The route could be compared to going on a spaceship or crossing into a parallel universe.
The Straight Road, so called because it follows the old path across Belegaer from before the Akallabêth when the Flat World was made Round, is only kept open to Elves, who are allowed to sail to it on their ships by a special grace of the Valar.
A ship departing on the Straight Road, when observed from the shore, would slowly become smaller to sight until it disappeared in a point, and not drop behind the horizon.
It is noted in earlier writings by Tolkien that some mortals besides those carried on Elven ships like the Fellowship of the Ring and Bilbo Baggins can also find this route in a Bermuda Triangle-like instance (like Ælfwine or Eriol from The Book of Lost Tales).