The stopper knot is used primarily in the mediterranean to more the boat with the ground lines given by the marina. The problem with ground lines is they need to be winched in to get the correct tension. The problem is taking off the tension of the line from the winch without losing the tension on the cleat so you can lock your line to the cleat.
This is done by using a stopper knot. These knots can be very dangerous if not done correctly, so practice with caution or have a professional show you in person how to do it.
Below is a brief video on a certain type of stopper knot you can do.