Something that's prorated is divided or billed proportionally to a specific share or amount of time — like a prorated hotel bill that charges you only for the night you stayed, rather than for the full week you booked before realizing the place was haunted.
The Latin root of prorated is pro rata, "according to the calculated share," or "in proportion." If you sign up for an annual cookie-of-the-month club in July, your membership fee should be prorated to charge you only for the remaining six months of deliveries. Prorating also applies to things other than time, such as dividing a company's profits among business partners based on the exact percentage of the company each person owns.