An active user is someone that is enrolled in your campus that logs in and accesses the campus. You may have users that are enrolled into your campus, but do not access the campus during the calendar month, these users are not considered "Active" and will not contribute to the active user count.
The active user report allows you to drill down further into your active users. Active users are important as they determine your pricing band. Check out the Plans and Billings section in your settings to see how many active users your plan allows.
To access the 'User Activity Report', click on the Users option in the navigation pane. The user's page will appear with tabs to the right, click 'Activity'.
Within this report you'll see a complete list of all your active users and all your inactive. You can export your active users list for your own records by clicking the 'Export' button at the top of the report.