Instant booking refers to properties where the calendar is kept up-to-date meaning you can book and pay for a property without having to wait for confirmation from the owner. It is really useful when you have found a good price for flights (or other transport means) because you can book everything at the same time.
Occasionally a property which is advertised as instant booking will need confirmation from the owner. You will be alerted to this whilst checking out. No payment will be required until the owner has confirmed the booking.