Complex sentence: The term "rocksteady" comes from a popular dance style that accompanied the sound, mentioned in the Alton Ellis song'Rocksteady'; dances performed to rocksteady were less energetic than the earlier ska dances.
Split into simpler sentences: The term "rocksteady" comes from a dance style that was characterized by slower movements, mentioned in the Alton Ellis son'Rocksteady'. Dances performed to rocksteady were less energetic than the earlier ska dances.