I don't understand it

The System.out.println() method calls .toString() on any object passed to it. This is happening on all the lines above that use .println().

Could you explain why is that? Is it specified in this method declaration?

The simple explanation is that it is just something that the println() method does. It is a very versatile method that you can pass literally any variable, object, class, primitive, etc to and it will print it to the screen. The more complex (but simplified) version is that the toString() method accepts any Object class and will call the .toString() method which is declared in the Object class. Everything in Java, including custom classes are objects that directly inherit the Object class, so are guaranteed to have a .toString() method whether you code it in or not. This is a simplified version of the actual reason, which books will devote a whole chapter to explain (plus there are a lot of other concepts to understand about class structure, inheritance, polymorphism etc). Don't worry too much at this point because you will be introduced to all these concepts as you go along.

If you want to see a quick example of this, remove the toString() in the Kaczka class and run the code. You can even call kaczka1.toString() and the code still works:

public class Solution {
    public static void main(String[] args) {
        Kaczka kaczka1 = new Kaczka();
        Kaczka kaczka2 = new Kaczka();

        // this still works even though the .toString() method has been removed
        System.out.println(kaczka1.toString());
        System.out.println(kaczka2);

        Pies pies1 = new Pies();
        Pies pies2 = new Pies();
        System.out.println(pies1);
        System.out.println(pies2);

        Kot kot1 = new Kot();
        Kot kot2 = new Kot();
        System.out.println(kot1);
        System.out.println(kot2);
    }

    public static class Kaczka {
        // remove toString() here
    }

    public static class Pies {
        public String toString() //(2)
        {
            return "Pies";
        }
    }

    public static class Kot {
        public String toString() //(3)
        {
            return "Kot";
        }
    }
}

The output will change a little though because you will be using the default .toString() defined in the Object class, which will print the full class name and hashcode. It will look something like this: Solution$Kaczka@5f184fc6