What does 'is an instance of' mean in Javascript?

Question

The answer to this question: What is the initial value of a JavaScript function's prototype property?

has this sentence:

The initial value

Answer 1

In JavaScript functions are first-class objects, that means you can treat them just like any object.

//Define class

   function User(name,age)
    {
        this.Name=name;
        this.Age=age
    }

//create insatnces

var user1= new User('Demo1','50');
var user2= new User('Demo2','100');


alert(user1.Name);
alert(user2.Name);

http://jsfiddle.net/praveen_prasad/hmUku/

http://en.wikipedia.org/wiki/First-class_function