Say you want to be able to access a web server in a remote unix but port 80 is not exposed to the world. Still, you have ssh access to that.
Here is what you can do:
ssh -N -L localhost:8080:localhost:80 -p 22 user@remote
Then you can access the page on port 8080 in your local computer:
Ta da!
Note that the second occurrence of the word ‘localhost’ will be evaluated on your remote computer. Then, the first localhost will point to your local computer, but the second to the remote. Same thing for the ports.
ssh -N -L local_address:local_port:address_in_remote:port_in_remote -p 22 user@localhost