Prorated charges are a partial charge for the time between starting the new service and your bill date. Subscription charges are billed in advance, meaning on the first month of a new subscription, you will be charged a prorated amount for that subscription plus one month for the next month.
For example, if a subscription begins on the 10th of the month you will be charged a prorated amount for the 10th through the last day of the month PLUS one full month for the next month.
These charges can be viewed under the "Subscription Details" section on page 2 of your invoice.