This is the documentation of JSU for Jira Cloud. If you are using JSU on Jira Server/Data Center, see our Server/Data Center documentation.
The 'Fields Required' checks whether values for specified fields are set for a given transition.
You must select mandatory fields when configuring the transition. For example:
A workflow is configured so that the 'Ready for Review' transition has the 'Fields Required' check where the 'Reviewer' is mandatory. It will then copy the Reviewer to the Assignee, assigning the issue to the reviewer.