It is a detailed checklist or blueprint of steps prescribed by the Head of Customer Success (CS) which should be followed during the customer journey. Also, it contains the best practices that you must follow while dealing with your customers to improve their overall experience with the product.
An onboarding playbook is, similarly, a detailed sketch of the onboarding process and the time required to complete the same. With playbooks, you will get a real-time view of which customers are in which step of onboarding.