The prisoners were kept in very poor conditions.

conditions 是名词 condition 的复数形式，condition 本义是"条件、状况"，这里指"生活或居住的环境状况"。 poor 在这里修饰 conditions，表示"恶劣的、糟糕的"，poor conditions 合起来指"恶劣的环境条件"。