The passcode is used for session validation. There are two passcode options, one is a 4-digit code and the other is touch ID (for Apple devices only.) If you have logged in with your password, you have created a new session. That session should last for 24 hours, if you do not close the app completely. If you switch back and forth between apps, you will be able to use a passcode to reenter the app. If you have closed the app or the session time has expired, then you will be pushed back out to the login screen.