The average # of seconds a daily active user spends in the app per session.
Session length is defined as the period of time when the user is actively engaged with the app. So things like switching apps without quitting the app, phone lock, even swiping down the notification screen will end a session. Background activity will not lengthen a session. A user must be in the app for at least six seconds for a session to be recorded.
This is the average # of seconds a daily active user spends in the app per session. This means that if a user has 2 sessions a day, their time spent in app = Avg Session Length x 2.