The Raven is probably the most famous work by Edgar Allen Poe; the gothic imagery, and the raven's "nevermore" statement have been endlessly repeated and paraphrased in other media.
While the poem itself is rife with symbolism, literary allusions and structural manipulations, the title itself is pretty straightforward; it is simply stating that the focus is upon the raven that appears in the story. It is the driving force behind the action of the story, and the most curious and unusual thing that appears in it, so it would be a suitable name for the work if for no other purpose than distinguishing it.
Ravens have featured prominently in mythology and folklore and are commonly depicted as wise, as well as having a connection with the dead. This may be an additional insight into the meaning of the raven in the story, with the title reinforcing our focus upon the raven's supernatural abilities.
The raven, long associated with darkness and death, is the mysterious messenger that prompts the persona to ask about his Lenore. Edgar Allan Poe's poem is more noted for its eerie and unnerving tone, but it is also a story of melancholy. The raven, a strange bird that suddenly appears out of nowhere, personifies the persona's sorrow - that he can no longer see Lenore ("Nevermore"). The raven is not just an outside reminder of his loss. It's a physical manifestation of his own futile attempt to convince himself that he will never see Lenore ever again, no matter how many times he asks the raven.
The raven, is not just a harbinger of death, but a mirror of grief, which is the poem's subject.