Imagine: you have 5 users in your team, whom you'd like to subscribe for the Professional plan.
Here you have two options: subscribe monthly or annually.
If you choose a monthly subscription, you will be charged $30 per user which is $150/month/5users (5*30=150).
If you go with the annual subscription, you will get a 20% discount, resulting in $24/user/month. This will account to 5*24=$120, saving you $30 a month and $360 a year!
If you decide to add a new user during the period of use (a month or a year), you will be charged the difference with the next payment.
Let's say you signed up for a year with 5 users and paid $1440. After 3 months you add the 6th user (they will use the system for 9 months before the next billing date). This way you will be charged for all the 6 users when the billing date comes, yet you will only pay the difference for the 6th one (for 9 months).
On the other hand, if you remove one of the 5 users during the subscription period (a year), the not-used money for their account will go to your credit balance. For instance, if they used the system for 3 months, you will get the rest, which is 9 months, credited to your account. You may use these funds for future payments.