If your server is configured to allow requests for domains that don't belong to any of the apps on your server, those requests will be handled by your server's default app (sometimes called the default site).
To enable the default app, go to your server's Settings tab and click the switch for "Deny requests for unknown domains" so that it is disabled (grey). This allows requests for unknown domains to be handled by your server's default app.
With this setting instead enabled (green), requests for any unknown domains are instead rejected by your server.
The default app is the app whose name is alphabetically first.
If you have two apps on your server, foo and bar, then the app bar will be the default because bar comes alphabetically before foo.
If you want to create an app that is always the default, create an app named 0default (a zero followed by the word default). As long as none of your other apps start with a zero, this app will be the default.
If you need to redirect requests from the default app to one of your other apps, enable the default app as described above and then create a .htaccess file in the 0default app's web root directory with the following contents:
RewriteRule .* http://example.com [R=302,L]