The expression 二八 (èr bā) literally means 'two eights' (2×8=16) and is a classical Chinese literary term specifically used to describe a girl of sixteen years old, an age traditionally associated with youthful beauty and grace. It is not the standard way to say 'sixteen' in modern Chinese (that would be 十六, shíliù); rather, it is a poetic or refined reference. You'll mostly encounter it in literature, poetry, or set phrases like 二八佳人 ('a beautiful girl of sixteen').