The MAU, as its name implies, is the number of active users in a month. They are the users who have opened your apps from the 1st of the month to the last one.
This means that it is the user, who when opening the application at least once a month is considered active. If the user opens two different apps from my account, they will be 2 MAU's. If you reopen them during the same month, they continue to be 2 MAU's.
To understand the new monetization model, we must understand what active monthly users are about, it is important to point out that the first day of the month begins with monetization until the last, then the account is reset and the process begins again.