Threshold(s) are bounds for metric values. For example a upper threshold for connection for autoscaling may be defined as 80% which is the trigger point of an scale-out Alarm.
Thus if the connections load goes over 80%, it would indicate a threshold violation and would result in the respective Alarm being triggered.
Threshold is encapsulated within an Alarm object as a part of the Alarm definition.
To see how to get or set alarm thresholds checkout Stream Manager API Documentation