"Biometric Login" is a catch-all term for authenticating a login to an app using a human characteristic - for example Fingerprint ('Touch ID') or Facial ('Face ID') recognition. You may see a different option depending on whether you're using the AAdvantage® Debit Card app on an Android or Apple device.
To enable logging in via Biometrics using the AAdvantage® Debit Card app, go to the "Account" tab and tap "Settings". In this section, you will see the option to toggle the Biometric login option that your device allows.
Keep in mind that you will only be able to toggle this setting if you have a Biometric ID set up on the device you're using to access the AAdvantage® Debit Card app. You can do this in the settings section of your device.