The hostnames the site is served on.
Manage permission only
This tab, even just listing the domains, needs the "Manage domains and credentials" permission. A domain decides which requests are let through to the site.
One domain is always primary
The primary domain is the one "Visit Site" opens. It cannot be removed or deactivated; to retire it, make another domain primary first. You can also tick primary when adding a domain, which demotes the old primary.
An inactive domain cannot become primary either; activate it first.
Removing a domain turns its visitors away
Once a domain is removed, visitors arriving on that hostname are refused. An inactive domain is treated the same way, except that it stays in the list and can be switched back on.
Each hostname can be added only once; adding one that already exists is refused. A hostname is saved in lower case, without a port or a trailing dot, and must contain a dot (or be localhost).
www is not counted separately
If you have added only example.com, www.example.com is accepted too, and the other way round. But if you add the www version as its own entry and deactivate it, it does not fall back to the bare domain; it is refused.
A new domain also affects Google sign-in
If customer sign-in with Google is set up, Google matches each domain's redirect address exactly. The Customer accounts tab lists the redirect address for every active domain. After adding a domain, register its address in your Google app; otherwise Google sign-in fails on that domain at Google, with no sign of it here.