Booking cutoff defines how many hours before your experience's start time it becomes unbookable on your online booking engine and by your resellers. If you i.e. select 12 hour booking cutoff time, customers and resellers won't be able to make a booking less that 12 hours before the experience starts.
Note, that your own employees can always book until departure via Booking desk regardless of a defined cutoff time.
Go to "Products" > "Experiences" and select the product you want to add a booking cutoff to. Navigate to "Availability" > "Booking cutoff" on the left hand side of your screen.
Select your cutoff time from the drop down menu and then select "Save & Continue"