PHP Namespaces let you group related classes, interfaces, functions and constants together, mostly to avoid name conflicts.
As of version 8 Drupal uses namespaces and follows the PSR-4 convention, which means your namespaces and module directories must follow certain rules to make sure class autoloading works as expected.
In this section you will refamiliarise yourself with namespaces in Object Oriented Programming, and learn how to use them correctly when working with custom Drupal modules.