The Training Fortune Cookie, once opened, grants 5 days of reduced training time at all three Neopian training schools (even that secret one). While the cookie is active, any Neopet you enroll in a course will not have to spend as long in the school before their training is complete.
Note that the cookie does not affect training cost! You still owe the training school masters the appropriate codestone/dubloon fee in order to train. We suggestion having these items ready in your inventory before you use the cookie to make the most of your discounted time!
Mystery Island Training School: http://www.neopets.com/island/training.phtml
Cap’n Threelegs Training Academy: http://www.neopets.com/pirates/academy.phtml
Secret Ninja Training School: It's a secret!