If you want til specify that a given port on the local (client) host is to be forwarded to the given host and port on the remote side. This works by allocating a socket to listen to port on the local side, and whenever a connection is made to this port, the connection is forwarded over the secure channel, and a connection is made to host port hostport from the remote machine. This could be useful if you e.g. are behind a proxy.
This is an example of forwarding port 2022, open a terminal and type
ssh -L 2022:<forwarded-host>:22 <username>@<remote-machine>
Now open another terminal and type
ssh -p 2022 <username-at-forwarded-host>@localhost