The Drupal hook system is a tool used to extend and customize the functionality of Drupal. It allows developers to modify the behavior of existing Drupal functions and add new functionality without having to modify the core code. Hooks are functions that are called at specific times during the execution of a Drupal request. They allow modules to interact with the core code and other modules, and to alter the page output.
Hooks are defined in a module's .module file. When a hook is invoked, Drupal searches through all enabled modules for functions that are named after the hook. If a function is found, it is executed. This allows modules to interact with each other and with the core code. For example, a module can use a hook to add a new field to a form, or to alter the output of a page.
Hooks are an important part of the Drupal development process. They allow developers to extend and customize the functionality of Drupal without having to modify the core code. This makes it easier to maintain and upgrade Drupal, as well as to create new modules.