It means "Hail Earendil, brightest of the stars." Earendil is a figure from the mythology that Tolkien invented as a background to LOTR; he carries the morning star, so is associated with light. Frodo says this sentence when he and Sam are trapped in the spider's lair in chapter 9 of the Two Towers; it's to do with the star-glass he had from Galadriel, I think.
