People were living in shacks built with metal sheets and bits of wood in those days.
=People were living in shacks that/which were built with metal sheets and bits of wood in those days.
那时,人们居住在用金属片和木片搭建的棚屋里.
此处的过去分词词组built with metal sheets and bits of wood in those days作后置定语,修饰shacks,意义相当于定语从句.