Complex sentence: Dewey Finn (Jack Black) is a guitarist in an unpopular rock band called No Vacancy, which has become downtrodden due to Dewey's arrogance and hyperactive stage antics.
Split into simpler sentences: Dewey Finn (Jack Black) is a guitar player in an unpopular rock band called No Vacancy. The band is generally dismissed and downtrodden due to Dewey's arrogance and hyperactive stage antics.