From time to time you may need to perform maintenance on your Drupal site. That's why the administration section of your site includes site maintenance settings.

Found at 'admin/settings/site-maintenance' these settings allow you to restrict access to the site and present a configurable message to users.

You will definitely want to use this feature when upgrading to a newer version of Drupal. It's also a good idea to use this when you are adding new modules to your site. By doing so you can ensure that the you maintain the continuity and integrity of your data. In the case of module upgrades you can protect your users from potential bugs and incompatibility while you take some time to test the module changes.