App_Offline.htm is a page that you can use to take your site offline when deploying updates or performing maintenance. The file allows you to completely shut down your website and route all requests to the app_offline page.
Use App_Offline.htm to show a friendly message to your users instead of an ugly error. You don't want the world to see your server exceptions, do you?
Create a file app_offline.htm and put it the root of your site (or virtual directory) in IIS. Apache users need to add a RewriteRule to enable the page:
RewriteRule /.* /app_offline.htm [L]
The file needs to be static (IIS) and cannot use any of your site resources (images, css, js) since all requests will be routed to the maintenance page.