Sites within Users will display any Sites that a user has access to due to creating the site or being invited.
Here, you can see more information about these Sites, such as:
Name - Name of the Sites
URL - URL of the Site
Owner - Client or Agency owner of the Site
Where can you find this?
From a list of Users within your Portal; click into a User by their name or pencil icon; this brings up the 'User Details' tab. From here, navigate to the 'Sites' tab.