Zeitwerk is the new code loader that comes with Rails 6 by default. In addition to providing autoloading, eager loading, and reloading capabilities, it also improves the classical code loader by being efficient and thread safe. According to the author of Zeitwerk, Xavier Noria, one of the main motivations for writing Zeitwerk was to keep code DRY and to remove the brittle require calls.
Read more about this at: Rails 6 introduces new code loader called Zeitwerk