This metric shows the unique number of users. Besides cookie information, also the Customer ID (for all users that are logged in) and the Cross Device information (if a login happened at other sites that use the Cross Device Bridge).
Compare this metric with the metric "Visitors" to see how many users are tracked more precisely by using the cross device bridge
This metric is only available when using the cross device bridge in preview mode.
Further information on the Cross Device Bridge can be found here.