As a reference point from Fabric's documentation.
Daily Active Users:
The number of unique app installations across all devices that were active on a specific day. Note: Foreground activity must happen for a device to be active.
Daily New Users:
The number of new app installations across all devices on a given day. Deleting and reinstalling the app would create a new daily user. Updating versions of the app will not create a new daily user.
Monthly Active Users:
The number of unique app installations across all devices that were active over the trailing 30-day period. Note: Foreground activity must happen for a device to be active.
Crash-Free Users:
The percentage of daily active users that had a crash-free experience on a given day. Note: On Android or iOS, caught exceptions or logged errors are not considered crashes and are not included in this metric.
Sessions:
The total number of sessions across all app installations on a given day.
If you go to the "Audience" page from your app's dashboard on Fabric, you can see the devices and OS versions being used for the current day. If you click on the graph for a day in the past, then you will see the values from that day.