He argued that a poem should be structured and crafted in a way that creates a unified, consistent mood and feeling throughout. This effect should be carefully chosen and planned, and all elements of the poem should work together to achieve it.
Poe believed that the single effect should be a "certain definite and single effect" and that "no part of the poem should be irrelevant to this effect." He even went so far as to suggest that any element that did not contribute to the desired effect should be cut.
While Poe didn't explicitly define the specific emotion, he often emphasized the dark, mysterious, and macabre in his own writing.
In essence, Poe believed that a poem should be a cohesive and well-crafted experience that leaves the reader with a single, powerful feeling.