In Drupal 7, to manage system variables, we used variable_get() / variable_set() / variable_del() calls and stored those variables in the variable table of the database.
In Drupal 8 we now use the Configuration system which provides a central place for modules to store configuration data. This system allows to store information that can be synchronized between development and production sites. This information is often created during site building and is not typically generated by regular users during normal site operation.
In Drupal 8 configuration is still stored in the database, but it can now be synced with YML files on the disk for deployment purposes aswell.