You can setup 2-Factor-Authentication for your account in your settings on "my Bitcoin.de → Settings → 2-Factor-Authentication", which replaces both the LoginTAN-method on login and the SMS-TAN-method for deposits and thus increase the security of your account if handled correctly.
Furthermore you are no longer dependent on your email provider for a login and you can conduct disbursements regardless of disturbance of mobile networks.
Basically there are 3 different types of 2-Factor-Authentication to choose from. If you want to use 2-Factor-Authentication via "Google-OTP" you need the "Google Authenticator" app (or a different RFC-6238 compatible application), that you should find in the app store of your smartphone.