A unique email address is just an email address that is only accessible by you and not anyone else.
The unique email address could be a personal email address or a business email address but the email address can only be accessible by you and not anyone else for it to be unique.
For example an email address such as
[email protected] would be a unique email address.
An email address that would not be unique would be something such as
[email protected] which would be accessible by other employees of yahoo.
But the unique email address is an email address that no employees use and only you.
In short a unique email address is your personal email address.