A GUID is an alphanumeric unique number(ID) that is provided by the EXAI server in order to differentiate all websites.
When you open your website created with EXAI, in the address bar of your browser you will find a very big URL something like (Some alphanumeric code).3.exai.com. This is the complete GUID of your website and the alphanumeric code is the ID of the sub-domain provided to your website.
The GUID is used to differentiate between websites and to provide a unique URL for all the websites until they are published. Once the website is published, there is no need of GUID.