Information on IRC Servers are at IRC Server
This parameter lets Mibbit know what server to connect to. In widgets, it connects to that server after the user had connected to Mibbit (including autoconnection). In the main client, it pre-sets the connect-form (if a channel is given as well).
When using the widget, some characters need to be "escaped" if you need to use them, such as : and +
The : character allows you to add a port. In a widget, it is used as %3A, for example
The + character allows you to add an SSL directive, In a widget, it is used as %2B, for example