You do not need a password to log into the app. You log in via a registration link that you will receive to your email inbox when you type in your email address in the login screen.
Your account is still secured as follows: The registration link in your login email only works on your device, i.e. logging in is only possible on the device from which the link was requested. Also, a link that has already been used cannot be used again to log into the app.
Should you ever lose your smartphone, we can block your account immediately. In general, payment data cannot be viewed in the app and bookings can only be authorized with a PIN that you have specified.