This trick allows to forward a service which can be accessed only inside a specific network to your machine (localhost).
Let’s suppose you have a specific server 192.168.1.7 which can be accessed only from the following machine 188.8.131.52 or another machine inside the 184.108.40.206 network.
Then, you type :
ssh -L3309:192.168.1.7:3306 firstname.lastname@example.org
And you can now access your service from localhost:3309 !
Note: If you leave the SSH connection, the port forwarding will be lost.