This relates to pricing differences depending on date and time selected.
Peak times are generally Thursday and Friday after 5 PM, all day Saturday, and Sunday until 7 PM.
Off-Peak times are generally all day Monday and Tuesday, Wednesday, Thursday and Friday before 5 PM, and Sunday after 7 PM.
Prime times are generally Friday at 7 PM, Saturday at 2 PM and 3 PM, and Sunday at 2 PM.