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:
JSU default behaviour is to apply the context settings, this means that a field that is required by this check, but having no valid context for the current issue, wont be checked. Thus you may select to ignore the context, in that case a field will be required by this check, even if its context is not configured for the current issue.
For information on how to configure a check in JIRA, see the JIRA documentation.
You can specify a custom error message, which might better explain the situation to your users. If you leave this field empty, a default error message will be displayed. However, that might be a bit too technical, making it difficult for you users to understand the validation error.
A workflow is configured so that the 'Resolve' transition has the 'Fields Required' check where 'Fix Version' is mandatory. If a user attempts to resolve an issue on this workflow, where the 'Fix Version' field its value not set, the check will fail and no action will be performed.
Supported Field Types
In its different modules (especially those for workflows), the JSU app supports many different field types. System fields, as well as custom fields.
However you should be aware, that not all field types are supported. Also not in all combinations. We think we cover the most important field types and still are continuously adding and improving which and how different field types are supported. But the one you need, might just not (yet) work. Some custom fields of other third party app might never get supported.
For that reason you should always test anything you do with the JSU app with fields. Before you buy a license for JSU, try it with a free evaluation license, if it works for you.