An anapest is a unit of poetry made up of two unstressed syllables followed by one stressed syllable. Some three-syllable words, like "contradict" and "interrupt," are anapests.
The structure and rhythm of a poem comes from its meter, the pattern made by stressed and unstressed syllables or "metrical feet." An anapest, a unit three syllables with the stress on the last syllable, is the opposite of the more common dactyl, which instead stresses the first of three syllables. "A Visit From St. Nicholas" by Clement Clarke Moore makes great use of anapests: "Twas the night before Christmas and all through the house..."