If you've modified or replaced the configuration file, nginx will not
immediately start using the updated nginx configuration.
Once a restart or reload signal is received by nginx, it
will apply the changes. The reload signal
$ service nginx reload
tells nginx to reload the configuration. It will check the validity of the configuration file and then spawn new worker processes for the latest configuration. It then sends requests to shut down the old worker processes. This means that during a reload nginx is always up and processing requests.