Subscriptions can be billed two ways, monthly or annually. With monthly subscriptions, payments are processed automatically every month on the anniversary date you signed up with. For example, if you signed up for a paid plan on August 13th, your plan will renew and automatically charge your card again on September 13th.
Annual plans work the same way, except it's billed on a yearly basis. Also with yearly plans, you save 10% of the monthly cost of the normal plan price.