I copy command from form Fortrabbit to access my database using a SSH tunnel:
#Access the database from outside using a SSH tunnel
ssh -N -L 13306:myapp.mysql.eu1.frbit.com:3306 [email protected]
I entered my passphrase. But when I hit enter nothing happens.
Could anyone help?

You have set up a port forwarding from your localhost:3306 to remoteport:3306.
What this means is the mysql database runinng on remote machine is now accessible at your localhost port 3306.
You can connect it using
Shell
JDBC
This will only work as long as the SSh session opened previosly is alive.