Every custom app comes with a whole set of analytics to gain valuable insight into the way your app is being used.
Every screen a user views within the app is an "interaction". This can be a form, a media item, etc.
A Session begins when the app comes to the foreground (i.e. is visible) and ends when the app goes into the background. Whether that's by closing the app or opening up another app on your device or locking the screen of the device.
Please note that Session length does not include time listening to audio while the app is in the background or the screen is locked.