An interface is simply an IP address that the FTP server is listening on. It can be an IPv4 or IPv6 address. The "Default" interface represents the settings that will be applied for newly detected interface of a particular type. When a new interface is detected, the new interface it assigned the values of the "Default" interface at the time of detection. There is a Default interface for each type of protocol (currently, FTP, FTPS, SFTP, HTTP, and HTTPS).
For example, If the "Default FTP" interface was defined to be on port 21, then when a new IP address is detected in the system (the administrator added a new network card or an IP address changed) an FTP interface with that new IP would be added and receive the port value of 21. Those settings then become the settings for the newly detected interface. Note that the new interface's settings are not linked to the "Default" interface in any way. The "Default" interface simply represents the values that newly detected interfaces will be initialized with. Changing the values of the "Default" interface wouldn't change any values on existing or previously detected interfaces.
When you first install Cerberus FTP Server, the "Default FTP" interface is set to port 21 (the default FTP listening port) and all interfaces detected during the first start will receive that value. If you later change the "Default FTP" interface value that change will have no effect on existing interfaces. The same rules apply for the Default FTPS, SFTP, HTTP, and HTTPS interfaces.
It is also worth noting that Cerberus remembers the settings for interfaces that were previously detected but might have changed. For servers that have dynamic addresses that constantly change or cycle between a range of addresses, Cerberus will "remember" the old values and apply those instead of the "Default" settings if that interface address is later detected again.