This metric will help you measure the level of engagement for your app. A user is active if it interacted with your app at least once in any form (check supported events) in the period of time specified.
The total will change if a new/existing user uses your app within the time range consulted for the first time, and the same applies on per-day basis.
It's likely to happen that the same user interacts with your app multiple days, adding one to the total, and adding one to the active users for those specific days. That means that on multiple day interactions, it will count as an active user for those days, though the total remains unchanged once increased.