When your server gets full, you will note delays when trying to connect to your server's primary listening port. Via the Port lines it is possible to specify additional ports (both AF_UNIX and AF_INET) for ircd to listen to. De facto ports are: 6667 - standard; 6660-6669 - additional client ports;
These are just hints, they are in no way official IANA or IETF policies.
On a side note, the /UPING command uses port 7007/udp. If your server is located behind a firewall, you may want to make another hole in it for this port.
The hostmask that is allowed to connected to the port.
Leave empty to allow everybody to connect.
It is the port number.
For what kind of connection the port will listen to.
Normal users will use the port.
The port will be used for the links with others servers.
The port is not listed in a /stat on the server.