Early on, the bird's "Nevermore" comes in answer to the narrator's request for the animal's name, but then it takes on a somewhat more sinister tone when the narrator,
mutter[s] "other friends have flown before-
On the morrow he will leave me, as my hopes have flown before." Then the bird said, "Nevermore."
The narrator is mourning the loss of his love, Lenore, and now he feels that this raven will leave him too, just as she did. However, the raven's "Nevermore" implies that he will never leave the narrator.
Further, though the bird is "still beguiling all [the speaker's] sad fancy into smiling," the narrator describes it as "grim, ungainly, ghastly, gaunt and ominous." He next interprets the raven as a "respite" from his sorrows and a chance to forget for awhile, but the raven says, "Nevermore," which the narrator interprets as a claim that he will never be able to forget his sorrows. At this point, he screams at the bird, calling it a "thing of evil," and he asks if there is any chance that he will meet Lenore in "the distant Aidenn" (Paradise or heaven). The raven's "Nevermore" now dashes the narrator's hopes that he might someday be reunited with his love.
Finally, when the narrator orders the bird to leave, insisting that it is something from hell, the raven replies, "Nevermore," and—to this day—it still remains atop the bust of Athena in the narrator's chamber. The light from the lamp throws the shadow of the raven on the floor, and the narrator says, "My soul from out that shadow that lies floating on the floor / Shall be lifted—nevermore!" In other words, the raven has now become symbolic of some kind of oppressive force working on the narrator: perhaps it is the idea that death is final and that there will be no happy reunion with loved ones once we pass away.