This firewall configuration tool uses ipchains to setup the firewall. You can also start this tool by typing lokkit in the shell. Once it is started you are given four choices. You can choose from High, Medium, No Firewall, and Custom.
High firewall blocks about everything. If you are using High you are severely limited in what you can do. You will not be able to run servers if you have the firewall set to high because the firewall will block all connections to the ports that run servers.
Medium is nice, but like High, you will not be able to run servers. I recommend not choosing either of these two.
No Firewall is the easiest for someone to run, but that leaves everything open. Being open is not always a bad thing if you are running a server. If the server is set to No Firewall then you will probably want to configure the firewall later using a tool like firewall-config.
If you do a custom firewall then you will be taken to another menu. On this menu there will be two sections. Trusted devices are ones that will not be blocked at all. Allow incoming has some common services listed by name and a space to put additional ports. Move the curser with the tab key and select with the space key. If you want to open a service, but do not know which port it runs on you can try looking in the /etc/services file and see if the service is listed.
When this utility is done it will store an ipchains configuration file at the location /etc/sysconfig/ipchains. You can go there and modify the file later if you like.