What does the message “Master key is unlocked” mean?
This notification is shown if you're using the Android version of Threema and have set a passphrase on your master key. In order to prevent the operating system from terminating the Threema process (which would cause the unlocked master key to be lost as it is only held in RAM, necessitating manual re-entry of the passphrase), the app needs to show a notification. This notification has no other purpose and is only required because of the aforementioned limitation of Android.
Please note: in times of extreme memory shortage, Android will even terminate processes that show a persistent notification. In that case, it is possible that you are prompted for the master key passphrase again. This is caused by Android; Threema has no influence on this process termination.