Complex sentence: The term was introduced in the 1950s to refer to teenagers who liked pop music and/or rock and roll, The term became widely used again in the late 1960s, when there was an increase in marketing of pop music and fashions aimed specifically at teenagers.
Split into simpler sentences: The term was introduced in the 1950s to refer to teenagers who liked pop music and/or rock and roll. "Teenybopper" became widely used again in the late 1960s, following an increase in the marketing of pop music and fashions aimed specifically at teenagers.