What does it do?
The integration tries to match JIRA users to Hansoft users in order to be able to properly sync user related information such as reporter, assignee, comments et cetera. Ghost users in Hansoft are free user objects generally used for planning. They are not full accounts in the sense that they cannot login and so, not perform any actions on the Hansoft side what so ever. In the event that your JIRA user base is larger than your Hansoft user base, the integration has the option to create Ghost users in Hansoft in order to properly sync the user information.
Why is this important?
In the scenario where an item synced in JIRA gets assigned to a user not found in Hansoft, the integration does not have a user object on the Hansoft side to replicate this action. It is strongly advised that you enable this option to ensure good user information on all synced items and avoid getting "missing matching user" notifications sent by the integration.