In computer terms a tunnel is a bi-directional connection between two networks. An example of such a tunnel would be when someone connects to their workplace from their home PC. They would use VPN (virtual private network) software to establish the connection.
This creates a secure connection over a public network, like the Internet. All the data is encrypted and secure from tampering. Another example would be when you make payments over the Internet. You will notice that the URL of the page that processes the payment starts with https://. This is an encrypted connection or tunnel between your browser and the web site.
Tunneling is also referred to as encapsulation. One type of network protocol is encapsulated by another. SSH and SSL are example of such secure protocols.