A polysyllable is a word with more than one syllable. In poetry, polysyllabic words often affect rhythm, sound, and pacing because they take more time to speak than short, single-syllable words.
Poets may use them to create a smoother, more formal, or more flowing line, or to fit a particular meter.
Example
The moonlit neighborhood awakened slowly,
its silver avenues and wondering children
shifting beneath the polysyllables of morning.