Advance booking period
This limits how far into the future customers can make bookings. E.g., if this is set to 90 days, a customer will not be able to make a booking 91 days from today. If the date is set to April 1st, a customer can not make a booking after April 1st or after 90 days- which ever comes first. You can do this through Settings -> {Venue Name} Customer Booking Options.
The multi line box allows you to place a custom message for visitors when they try to book past the advanced booking period. The standard message will show how many days in advance they can book.