What happens here ?

This is the homepage. As no real URL-route (like /register/index) is provided, the app uses the default controller and the default action, defined in application/config/config.php, by default it's IndexController and index()-method. So, the app will load application/controller/IndexController.php and run index() from that file. Easy. That index()-method (= the action) has just one line of code inside ($this->view->render('index/index');) that loads application/view/index/index.php, which is basically this text you are reading right now.