When the speaker of this poem first encounters the raven, he does not think too much of it. In fact, he thinks it is a tame bird that has escaped its home and is looking for temporary shelter. The raven's continued uttering of "nevermore," however, soon begins to anger him. On its own, the word is meaningless. The speaker wants the bird to explain what it means, "to hear discourse so plainly." But all the raven ever says is "nevermore."
This reminds the speaker of his lost love, Lenore. His growing agitation at the raven shows the weak condition of the speaker's mental state. At first, he thinks that the raven is something that can distract him from his sorrows. However, he interprets the bird's response of "nevermore" as meaning that he can never escape these haunting feelings of loss and heartbreak. He is still tortured by the loss of Lenore, and he comes to think that the raven is mocking him. He concludes that the bird is a demon sent to torment him. He commands it to explain itself but only receives more replies of "nevermore."
Of course, the raven says nothing about Lenore. It is the speaker himself who draws the connection, highlighting his preoccupation with loss and death. The repetition of the word just drives this feeling deeper into him. Perhaps he wants the bird to say something different, to explain that things will get better. Yet, the raven remains perched above his door as a nearly silent reminder of death.