thread safe singleton using enum

I am not able to understand how Enum can be used for thread safe singleton instantiation. So let us say I have a class A which I want to make singleton. How do I do that using Enum ? I came across suggestion like the code below, but not really able to understand. In the below shall I replace INSTANCE with A singleObj = new A(); ?

Moreover, how exactly should class A look like as in what thing should I take care of there .. for example: making constructor private etc.

public enum EasySingleton{
    INSTANCE;
}