When a user unsubscribes in your site, it means that he is removing his email address from your email list. It means that a user no longer wishes to receive update emails from your site.
What is an unsubscribe rate?
An unsubscribe rate is that which measures the number of users that has opt to to.receiving emails from your site.
How is the unsubscribe rate calculated?
(Unsubscribed number/Emails delivered x 100 = Unsubscribed rate %)