Firewall: Trusted Services
Desktop Training - Fedora

The trusted services on a firewall are those applications that you have selected to allow to connect to your computer.   In the graphical interface window you can see the ones enabled are checked while those not enabled are blocked.

Lesson 4 / Lesson 6

Regardless of skill level you can enable a service by placing a check mark and then clicking "Apply".

trusted services

Here is an example of selecting FTP.  This would be an option that you would select if you had a FTP account you wanted people to log into your machine, Desktop or Server, and allow them access to files.  This connection would be made on port 21/TCP.  Be careful which services you allow as this is the area that will get you in trouble.