Threshold(s) are bounds for metric values. The default upper threshold for connection for autoscaling is defined as 60% which is the trigger point of an scale-out Alarm. Thus if the connections load goes over 60%, 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.
For example, if your node's connection capacity is set to 100, and the alarm threshold is 60%, creation of a new node of that type will be initiated when the total number of connections reaches 61.
To see how to get or set alarm thresholds checkout Stream Manager API Documentation