The free 3-day trial does not cost anything and is completely free for 3 days. It will give you access to Math Learner. The warnings that popup when you are getting the trial subscription are stating that once the free trial expires, it will become a paid weekly subscription if you do not cancel your trials subscription at least 24 hours before the expiring date.

Always carefully read all popups and only proceed if you agree with all popups. By proceeding you are agreeing with all terms.

If you do not wish to continue with the subscription, you can interrupt the service anytime on your iTunes subscription settings.