The Error Stack Trace window becomes available when you have an error. It provides information about the errors by displaying the function call hierarchy leading up to the error making it easier to troubleshoot and identify the root cause.
You can access the stack trace from:
the error message
the Annotations bar at the bottom of the Translator
The stack trace provides the function name and line number. You can click on the function to navigate to that function within the script and alter the annotation block of the destination function to show the relevant iteration.