SSH tunneling or also known as port forwarding is a way of forwarding normally insecure TCP traffic through SSH. Common ports for example POP3,SMTP,HTTP and FTP can be easily tunneled using SSH.
SSH tunneling is also sometimes used to bypass firewalls where certain ports are blocked.
The figure below represents the network setup at my workplace where the only port available to use is SSH port 22. By implementing portwarding over ssh I am able to port forward Oracle traffic over port 22 to my Oracle DB database running on my private VM with host only networking enabled.