Originally, the "fashion" of Middle-earth was the flat world of the mediaeval universe. Valinor (the equivalent of Heaven in that the "gods" dwelt there) was physically connected to the rest of the world and could be reached by ship. When Númenor sank "the fashion of the world was changed": the flat world was bent into a round one, with new lands also being created; and Valinor was removed "from the circles of the World", and could no longer be reached by ordinary physical means. The Elves alone were still allowed to make a one-way journey to Valinor along "the Straight Road". (An elven ship on such a journey would grow smaller and smaller with distance until if vanished rather than sinking over the horizon as a human ships do.)
References to "bent seas", "bent skies", "the straight road", "straight sight", "the World Made Round", and the like all refer to the change in the world's "fashion". (The palantír at Emyn Beriad "looked only to the Sea. Elendil set it there so that he could look back with 'straight sight' and see Eressëa in the vanished West; but the bent seas below covered Númenor for ever." (RK, p. 322)