When signing up for the Plus Plan, you have two options:
Monthly: Pay $15 each month
Annual: Pay $120 for the year
When choosing the Annual plan, you save the cost of 4 months' subscription fees under Monthly. A year of Monthly service is $180 (12*$15), but Annual is only $120.
The $60 you save is equivalent to getting 4 free months!
Important: all subscription are priced on a recurring basis, in order to stop automatic renewal you will need to cancel the subscription.