Complex sentence: With the arrival of a "progressive" label, the music was dubbed "progressive pop" before it was called "progressive rock", with the term "progressive" referring to the wide range of attempts to break with the standard pop music formula.
Split into simpler sentences: With the arrival of a "progressive" label, the music was dubbed "progressive pop" before it was called "progressive rock". "Progressive" referred to the wide range of attempts to break with the standard pop music formula.