An important concept in Drupal is the concept of module hooks. A module can expose one or more hooks empowering other modules to modify its behavior.

This means that if a module doesn't do exactly what you want, you can use a hook to "hook into'' the process and change that module's behavior without having to change the original module code. Moreover, it means that when the original module whose behavior you have overridden gets updates, you'll be able to reap the benefits of the update while still keeping your modification(s) in place.

An example is hook_entity_update—a hook that is called after anything (any entity) is saved. Other modules may want to perform an action of their own in response to an entity update, and so we will also implement this hook.

Implementing hooks in your own modules is beyond the scope of this this module. However, we'll see more on hooks in Module 2, Drupal 8 Development Cookbook and Module 3, Drupal 8 Theming with Twig of this course, but it's important to understand the terminology that you will no doubt come across as you continue working with Drupal.

For much more detail about the available hooks in Drupal, visit https://api.drupal.org.