An anonymous filesystem is a filesystem that is not bound to a domain. Even anonymous filesystems have a domain that can be used to print the absolute path of its files, but you can't use that domain from outside the anonymous filesystem to access it.
It is a temporary filesystem that only exists for the duration of a transaction.
An alternative way of thinking is that an anonymous filesystem is not mounted anywhere. As soon as you mount it to a non-anonymous filesystem, it will stop being both anonymous and temporary.