Poe's works are filled with imagery and symbols. For example, the title of the poem is the Raven. To me, the Raven symbolizes sadness caused by solitude and separation. There is the famous line:
"Quoth the Raven, Nevermore."
This line is repeated over and over in the poem. Technically in poems, this is called a refrain. A refrain is a line that gets repeated again and again