Jira workflows consist of statuses and transitions. A transition is the change from one status to another. JSU gives you a lot of power to modify the behavior of your workflow as it transitions.
Here are the parts of a transition you can configure:
Conditions decide if a workflow transition is available for the current user.
Validators check the field value of the issue.
Post-functions modify data in Jira.
This is the place where you will find JSU.