Prorated bandwidth means the bandwidth allocated for a portion of a month. Suppose the user is ordering for a service that contains 1TB bandwidth on 5th April then he only needs to pay for the remaining 25 days. So the user will not get 1TB bandwidth completely. It will allocate based on the remaining days.
1 TB x (25 days he paid / 30 days in April) = 853 GB.
So the user will get 853 GB BW for the month.