SSL (Secure Sockets Layer) is a method of encrypting your connections on the Internet, so that your connections are still safe if they are being "eavesdropped" on.
Setting up your client for SSL
mIRC's official documentation can be found here.
SSL can be used via mibbit by clicking on the "Server" link on the connection dialogue. or by using "+6697" as the port. Note also that if you wish to use SSL with mibbit, you may also wish to use https (https://chat.mibbit.com/ instead of http://chat.mibbit.com)
Rizon's QWebIRC automatically uses SSL when connecting via https
XChat, irssi and other IRC clients tend to have SSL support bundled in already.
Connecting with SSL
Typically, your client will have a connection dialogue which may ask you if you wish to connect with SSL as one of the options, allow you to pass a -ssl argument as part of the connect command, or use a "+" in front of the port number (i.e. "/server irc.rizon.net +6697", "/connect -ssl irc.rizon.net 6697")
Currently all Rizon servers have SSL support on port 6697 and 9999 (some servers also having other ports open for SSL).
Rizon BNC allows SSL connections via port 12345 only.