Pro-rated services are divided based on another value, usually a unit of time. Common examples of things that are prorated are rent or utility bills, which are reduced based on the amount of time you have used the server. If you start your server mid-month, your bills are usually prorated to take into account that you only used the server hosting service for a percentage of the regular billing cycle.
Your server rent is $30 a month, and you rented the server on 11th of the month. You would only pay From 11th of the month to the 30th of the month, which would come out to $20.00, since you are only paying for 20 days of the service.