What's happening here?

The Person class has a field (class variable) : which is initialized to 20 (when a new object of Person Class is created).

public int age = 20;

The Person class has a method:

public void adjustAge(int age){
           this.age = age + 20;
           System.out.println("The age in adjustAge() is " + age);
       }

, which takes an int as a parameter. This method takes the age Field which is created with the current instance of the class:

this.age

and adds + 20 to this value.

this.age = age + 20;

Now in the main method, as you create a new Instance of the Person Class:

Person person = new Person();

With this you have a new instance of the Person class, called person. This person instance has the int field named age, initialized to 20, which we discussed above. In the method we referred to this variable as:

this.age

But from outside of the instance we refer to it as

person.age

Now, In this Line:

person.adjustAge(person.age);

you call the adjustAge method from the person instance ( person.adjustAge), and we pass the int Field of Person Class as Paramter to it ( person.age), because the method requests an int Parameter. I hope you understand it this way. If you have not done it, I suggest U install Intelijj with the codegym Plugin, these things will be much more clear using an IDE (in this case Intelijj.)