When you cancel your subscription, it will remain active until the end of your billing cycle. You will not be charged at your next billing cycle.
In other words, you will stay be able to benefit from premium features until the end of the subscription period, but you will not be charged again after it ends.
For example:
You upgrade to a monthly subscription on 1st of January, then after 20 days you decide to cancel your subscription, you will still have a premium subscription until the end of January but you will not be charged after that.