According to the various famous verses from the poem that, to this day, many can say with their eyes closed:
Once upon a midnight dreary
While I pondered, weak and weary
Upon many and quaint a curious volume of forgotten lore
and then he moves on to say that, he was nodding and napping.
Basically, the main character and narrator was sitting by the fire in a dark Winter night, pondering in weakness and feeling weary.
This, he is doing because he is in mourning of his love, Lenore. Since he feels so lonely and sad and the night's weather is making things apparently worse for his psyche, he resolves to simply stay inside, think about Lenore, and allow nostalgia to take over his mind and heart.