Silverman quoted Poe saying that the meaning of the Raven is "Mournful and Never-Ending Remembrance", that is, a memory that will not go away and is stricken with grief.
The protagonist is in obvious mourning, and misses the love of his life, Lenore. He is also apparently fighting a battle of whether to move on, or continue to remember what once was. Hence, he has the same battle with the Raven. See how he wants it to "go away" but still seems enthralled by it; It is the question of "should I let go of this pain and memories, or should I retain them?"
The presence of the Raven at the end of the poem means that the protagonist chose to cling to memories, and live in state of mourning for good.