Some of you probably already know this, but in case you don't, I discovered this yesterday.
SSH (or Secure Shell) is basically a way of controlling your entire server from the command line. Root users can install and uninstall software, change configuration settings, and even remote reboot the server.
However, by default, all it requires to access it is a username and password that has Shell Access (normally the root user). If someone discovers, or sniffs out, those login credentials then they have instant access to your server. This is not what you really want is it?!
So, if you have WHM on your server (this is just an example...you can do it without WHM), you can login as the root user and set your SSH security to require an SSH key file in order to work. You'll still have to enter your username and password but you will also have to upload your key file into the SSH remote console in order to login otherwise even the root user won't be able to log in.
This will increase your security and reduce your chances of someone hacking your server over SSH. This is a basic example of protection.
Of course, there are loads of other ways you can make your server more secure. The best one, perhaps, being a firewall. ConfigServer is a good one and it's free. What's more if you have cPanel or WHM it has a special UI that you can control the entire firewall directly from WHM via the plugin it installs. This makes life a lot easier as you would otherwise have to set it up from the command line.