What does the raven symbolize in The Raven poem?

The Raven could signify two things, one dark and one light.

In darkness it could signify is the dark and depressing memory of the loss of Lenore, constantly reminding the writer that he shall never see his love again.

In light it can signify the release of the memory by a means of death, which in turn will reunite the writer with his love Lenore, as common folklore claims Ravens carry away the souls of the dead (And crows as well for those that will say "No it's crows" because you saw the movie).