In any of the mIRC options dialogs, you will see a Help button, which will give you more info. In this case you would see:
Advanced...
The random local ports option makes mIRC use non-consecutive ports when creating sockets, this might help against DoS attacks trying to disable your server or dcc connections.
The bind sockets to IP option makes mIRC bind all sockets to a specific local IP address, in case you want it to use a different outgoing network connection.
