See the section further down on the page: Cross-Origin Resource Sharing (CORS)
If you initiate a trigger from a web page hosted outside the SnapLogic domain, the browser enforces Cross-Origin Request Sharing (CORS) access control to secure cross-domain data access. See Cross-Origin Resource Sharing for more information. Since the web page making the request is not hosted on the SnapLogic domain, the cross-origin request will fail. However, if you configure the list of domains that can make cross-origin requests to the trigger, then those domains will have access to it.
The procedure under Managing the CORS Whitelist explains how to set that up.