This widget is the second Widget and is represented by the orange bar graph button. 
At a glance it gives you a summary of how many times users have logged into the database. The Bar graph represents the total logins for each day over the last 7 days.  The number displayed is the total logins for yesterday.
When you click on this widget, you will be directed to the Manage User area. This is the same screen that you access by clicking the User Widget.  
Next : Last Backup Widget