Authorized domains allow you to specify which domains will show your placements. This is a safety feature to prevent unauthorized use of your placements.
To add an authorized domain, go to the “Domains” page by clicking the link in the sidebar. Once you are here simply add the name of the domain without http:// or www. So if you were adding http://www.test.com as an authorized domain you would just add test.com. Then save your changes.
To remove domains from your authorized list simply remove them from the list and save your changes.
Placements will only show on domains in this list. They will also show on any subdomains such as app.test.com or help.test.com as per the previous example.