Linux offers a standard set of services active from the box. These include web-server, telnet, ftp etc. These are normally enabled as default. It may be hard for beginners to determine what these services offer and why they should be inaccessible from the internet.
This firewall first determines which these services are by looking up listening services. This list is presented with a button to click if the service should be accessible from the internet.
If the computer offer no services to the internet it may still be a good idea to at least enable one service. This is 113/auth as this will enable other systems to determine who you are when you make connections to them.