At the beginning of the poem, the word "nevermore" is just a response by the raven to the questions of the narrator. However, as the "conversation" continutes, the word becomes an expression of the narrator's own feelings, even though it is still spoken by the bird. The narrator is grief-stricken, and feels that his grief will last eternally. It will "nevermore" go away. The word becomes a symbol of the narrator's internal conflict.
Poe, in "The Philosophy of Composition", explained that grief was his chosen theme in this poem. He chose the word nevermore because of the strong "o" sound, feeling that this particular vowel best expressed a feeling a sadness.
The link for this essay is below.