What is an ssh tunnel and how does it work?

Here is the article. 

SSH tunneling, or SSH port forwarding, is a method of transporting arbitrary data over an encrypted SSH connection. SSH tunnels allow connections made to a local port (that is, to a port on your own desktop) to be forwarded to a remote machine via a secure channel.

To protect our network services, not all of them are reachable directly from outside the ENCS network. If you are offsite and need to access a resource that is protected in this way, you can use ssh to tunnel through an accessible resource to reach the protected resource. We generally recommend using the host "tunnel.encs.concordia.ca" for this purpose.

For more detailed information, please read more about the "-L" parameter at the SSH man page.