...
$this->request()->get('req3') is used to get the third value in the url. For example: [http://YourSite.com/index.php/req1/req2/req3/]...
In this case, current url is: [http://YourSite.com/index.php/to-do-list/view/todo-id/todo-title], so
request()->get('req3')will return the id of the Todo List
...
it will redirect you to the controller todo.view that you had registered in start.php, that means you will be redirected to the url: [http://YourSite.com/index.php/to-do-list/view/todo-id/todo-title], thus, the route for todo.view controller has to be 'to-do-list/view/:id/:name' in order to redirect and pass request parameters correctly.
...