You will be asked to insert your payment details before sending a booking request.
Your credit card will not be charged then; it will only be charged once the landlord accepts your booking request.
In case you sent more than one booking request, the first request to get accepted will automatically be your confirmed booking. All the other pending booking requests will be instantly declined and you will not be charged for them.
For more information on how your money is processed, please contact your bank provider.