With the Perform section, you can set up the individual commands to run on the network selected in the drop down list.
By default, that list starts with an All Networks option, which means that the commands would be sent to whatever network you connected to.
Commands would be similar to things like
.msg nickserv identify <password>
Which would send the command to identify you to nickserv when you connected to the network. If the network selection is All Networks, then it'll send that information no matter what network you connected to, even if it's one that you haven't registered with.
This is a very simple example. Additionally, using the Perform section sends the command(s) as soon as you connect, thus if the network isn't ready for the information yet, the network might not recognize the information because it was sent too soon.