The number of daily unique users with successful authentication or authorization activity. You can see the number of unique users for each day by hovering your mouse over a point in the graph.
The percentage of users that were active during the given time frame, calculated from the number of active users out of the total number of users on the tenant.
You can view data for a given time frame by using the datepicker next to the Compare to last period checkbox. There are builtin date ranges for the last 7/14/30/60 days, or you can provide the From and To dates for a custom range.