When you sign up for the yearly plan you purchase an entire year of Marvin. This is why you get the discounted rate compared to the monthly subscription.
You can cancel your subscription anytime which means that it won't repeat automatically.
In a yearly subscription, the repeat cycle is every year. So your subscription will simply run out after the year is up and not renew. But it does not change the repeat cycle and stop your subscription for the year you already paid.
Therefore, you don't get any money back as you paid for the entire year. And the discount applied because you committed for an entire year.