You can change what the customer sees on the booking form when they make a booking.
Navigate to the settings option, and select the 'Booking Form' wizard.
This setting is found on the first tab 'Design'.
To edit your success message scroll down to 'Would you like to display a message to your customers upon making a successful booking?' and edit the text below e.g "Thank you. We have received your booking."
Click Save at the bottom of the box.