Alliteration is the repetition of innitial consonant sounds.

For example:
During the whole of a dull, dark, and soundless day in the autumn of the year…”

The effect of an alliterative phrase draws attention to specific words and often the connection between those words; it can also create rythm to a piece of literature. In this specific example, the repetition of the "d" sound contributes to the overall opressive atmosphere Poe establishes with the opening paragraph of his short story.

(Excerpt taken from "The Fall of the House of Usher" by Edgar Allen Poe. For the complete story visit http://www.bartleby.com/195/10.html. )