Can someone please explain?

A setter method allows you to set the value of an object's private variable. So for example if we had a Car class that had a private string Model, the only way to change the value of Model of a Car object from outside the class would be to have a public method called setModel that changes the value. Then you would call setModel("Ford") or something like that from outside the class.

But if you simply don't make a public setter method for a private variable, then there's no way to change it from outside its class so it's considered immutable.

So in my example if setModel doesn't exist, then there's no way to change the object's Model variable from outside the Car class because Model is private.

Also, as an addition to what Silviu said—and is spot on!—if I understand it correctly, you don't want to use the final modifier because that means whatever value is first assigned to those variables can never be changed, and it's not a variable that the user is to directly interact with.

I came across this article, and it explains the different kinds of modifiers and how you use them: http://www.studytonight.com/java/modifier-in-java.php

So, if you have a variable that will never be accessed by the user of a program, but just by the program itself, and you want the value of that variable to be permanent, then you use the final modifier.