Drupal 7 updating image styles
When Drupal wishes to allow intervention from modules, it determines which modules implement a hook and calls that hook in all enabled modules that implement it.
The available hooks to implement are explained here in the Hooks section of the developer documentation.
By the end of this tutorial you should have a better understanding of what the migration system is capable of and know where to find more information about how to use it.
Learn more about the role that each fulfills in the Core Migration Modules tutorial, as well as plugins contained by other core modules that make use of the migration framework to ensure the content or configuration they handle has a migration path.
Drupal's module system is based on the concept of "hooks".
A hook is a PHP function that is named foo_bar(), where "foo" is the name of the module (whose filename is thus foo.module) and "bar" is the name of the hook.
Whether you're updating from Drupal 6 or Drupal 7, or importing data from some other source, you need to know about the migrate system in Drupal 8.
This tutorial provides an overview and links to additional tutorials where you can learn more about how all the individual parts work.