Mediator Pattern Advantage

Im reading the book of GoF. Could you please explain the following Advantage:

It limits subclassing. A mediator localizes behavior that otherwise would be distributed among several objects. Changing this behavior requires subclassing Mediator only; Colleague classes can be reused as is.

Is it meant that we should subclass the Mediator or the ConcreteMediator? Can we have more than one ConcreteMediators that inherit form the same Mediator?