This cloud wiki setting specifies a URL that should precede any internal links in the wiki, and can generally be left blank unless the wiki would normally be accessed via a reverse proxy, see Apache 2, Reverse Proxying and Your Wiki for an example of this setup and why it can improve wiki performance.
You should reboot the server after changing this setting.
Cloud Bug: Altering the Base-Url on a living server will cause cached content to contain bogus links referring to the old base url. To fix this, you will have to use the sqlite utility and enter the following:
DELETE FROM CloudHtml;
This will flush the cached html, which will be generated next time the wiki displays the content.
This node is a part of the cloud wiki manual.