In order to contact a host, you have to make a booking request. This does not commit you to anything, only to obtain more information about your possible trip.
If you register your credit card while making a booking request, you accept the automatic debit, that means you'll be charged automatically for the first host who accepts your request.
So, if you find any other place to sleep, don't forget to cancel your booking requests first before booking anywhere else (hotels, hostels, etc.) !