Practically, it says that in order to deal properly with the diversity of problems the world throws at you, you need to have a repertoire of responses which is (at least) as nuanced as the problems you face.

Ross Ashby, a pioneer British cyberneticist and psychiatrist, formulated his law of requisite variety in the context of regulation in biology — how organisms are able to adapt to their environment — and then, in quick succession, to aspects of Claude Shannon’s information theorem, and systems in general. Such interdisciplinary bridges were characteristic of the cybernetic approach. Stafford Beer extended the concept to help analyse the structure and management of organisations and whole societies.