Buffer time is the minimum time how much in advance customers can book. 

For e.g., if the current time is 10:00 am and you have set a Buffer-time of 3 hours, the first available/bookable time the customer will see is 1:00 pm.

By default, the Buffer-Time in set to zero. However, your business might be so that you want an advance notice to prepare in certain situations. This gives you control over that time.

You can change the value of Buffer Time in your account by going to 

Account > Settings > Booking > Buffer Time