The addressbook contains a list of I2P domain names and their Destinations (the I2P equivalent of an IP address).
Every domain name corresponds to a service. Some of these domain names may be for websites; some are not. Many of them are now defunct; when the original operator takes down a service or loses its encryption keys, the corresponding domain name is lost.
By default, the addressbook is subscribed to several I2P domain registration services. The domain names on the "Default" list are all from these services.
You can manually add domain names on the "Private" list.