How does check of authentication work in laravel?

I have the following line of code in my controller:

public function __construct() {
      $this->middleware('auth');
}

So basically what this line of code does is it checks before any method in the controller is run that the user is authenticated , I have a little confusion about this line of code though , what i want to know is where is this property middleware coming form ?? and how does this this method middleware really get included into my controller ??

Thank you.

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