SSH is an encrypted form of telnet, where everything you send from your machine to the server is scrambled in such a way that it is near impossible for people looking at your network traffic to determine what you actually sent.
There are two main reasons for wanting to use SSH instead of your normal telnet or mud client.
To play Nanvaent with an SSH client, simply configure it to connect to nanvaent.org. The username you should use when connecting is nan. The password is the same, nan. When successfully logged in, you should get the familiar Nanvaent login screen and can login to the actual game by using your player name and password as you normally do. That's all there is to it.
You can use the Java SSH client that takes you straight to the login screen.
The recommended ssh client for UNIX is OpenSSH, as it's small, secure and installs without too many hassles. However there are dozens of others available, some more fancy than others.
You can also use the Java SSH client that takes you straight to the login screen in a web browser.
A good resource for finding free ssh clients is the www.FreeSSH.org site.