What is meant by "advance bookings release time"?

This is the time when bookings will be available on the day in advance to what is set.

Example: Your advance booking for activities is set to 6 days. A customer attempts to book on a Wednesday at 2pm for a class the following Tuesday which starts at 4pm. However, as it is further than 6 days away in terms of hours they are unable to book. If the advance booking release time was set to 8am, all of the following Tueday's activities would be released from 8am on the Wednesday. 

1. Go to Settings > Bookings. 
2. Enter a time in to the Advance bookings release time box in 24 hour format.
3. Click Update details at the bottom of the page.