Encryption is a method of protecting data's confidentiality. When data is encrypted, it becomes unreadable to any unauthorized persons who get access to it.
If you send notifications to your Fitbit tracker from an Android device, some notifications may be unencrypted. If we’re unable to encrypt a notification, which happens periodically on various devices, you'll be alerted and given the choice to retry the encrypted transmission or to allow the transmission to pass unencrypted.
Note that unencrypted in this context means only that the text visible on your tracker is potentially vulnerable to an eavesdropping device. So, a person attempting to intercept Bluetooth transmissions might see the contents of the message, or if you received a call notification, the phone number. Your personal information stored with Fitbit is always encrypted. You can require that all notifications to your Fitbit tracker are encrypted.
For more information see, How do I get notifications from my mobile device?