Where do I find it?
Configuration > Transportation > Manual/Flyer/Calculated Discount
What does it mean?
The prebooking option is used to make a discount valid just for a specific period of time before the event.
For example, if you have an event at 17:00, you can create a discount just valid for 60 minutes before the event. It means that the discount just will be applied from 16:00 until 17:00.
What should I do?
The prebooking option needs to be activated when you are creating a discount (manual, flyer or calculated discount). However, in this case, you need to enter the minutes before or after the event depending on the period of time that you want to make the discount valid.
In this case, if you want the discount valid from 16:00 until 17:00 for an event at 17:00, the "Relative Prebooking Start" would be 60 minutes (before the event) and the "Relative Prebooking End" would be 0 minutes (it is when the event starts).