what does this usage of apply() means in Javascript

Please, can someone tell me what does this.init.apply(this, arguments) do in the code below?

I understand what apply() does in general, but in the context of the code below, what is it doing in there?

var Class = function() {

    var klass = function() {
        this.init.apply(this, arguments); //I don't really get this bit...
    };

    klass.prototype.init = function(){};

    return klass;
};

var Person = new Class;

//Usage
var someone =  new Person;

I see a lot of people using it. I've got an idea of what it does but can't really put my hands on it so I need more light.

I'm going up an extra level in JS, so I wanna know everything about it, not just the simple 'Hello world' level.

Many thanks