Overview
Trusted domains are set by websites to specify which domains are allowed to create and load new threads with the Disqus javascript embed. As a site owner it's highly recommended that you set at least one trusted domain. It's otherwise possible for any website to load your Disqus shortname and contribute comments and threads.
Note that it is not possible to retroactively set a trusted domain on existing threads. The domain will only apply to newly created threads.
Setting a trusted domain
Go to your Disqus Admin > Setup > Advanced. Locate the "Trusted domains" box and enter your domains there.
Best practices and troubleshooting
Sub-domains are unnecessary, as everything is covered in the base domain. So using
example.comwill coverblog.example.comandm.example.com.Don't include
wwwin your trusted domain, or else your comments may not load.As part of the Best Practices for staging sites you can use the trusted domain as a check to make sure you don't load accidentally load a production Disqus shortname on a staging site and vice versa.