"Natural law" means law that is not made by a deity, or by a government, but comes directly from the nature of humanity. Be aware that you are reading some kind of advanced historical and theological material. The writers may have some technical definition in mind. The exact definition may be understood by people in the field. Maybe they explain it somewhere else in the text. Maybe they are about to explain it when they explain "collegialism." Broadly, one example of natural law is the UN Universal Declaration of Human Rights. It begins: "recognition of the inherent dignity and of the equal and inalienable rights of all members of the human family is the foundation of freedom, justice and peace in the world." It says that "all members of the human family" have "inherent" dignity and rights simply because they are human. Where do these rights come from? Just from the nature of humanity. It goes on to say things like "Everyone has the right to life, liberty and security of person." Why? They just do. (Note that no deity or religion is mentioned). It is not sacred law, it is not judicial law, it is natural law.