Calculated fields are fields whose values are dynamically calculated either based on other fields of the issue or fields of other issues (such as sub-tasks or linked issues). You could use these fields to pull in data from an external system and display them with/without a calculation. The value for these fields is calculated every time the issue is displayed, or updated or a re-index is performed. You can search issues for this custom field in the Issue Navigator, in a Saved Filter, in an Agile Board query etc. only when a Search template is configured for the field.
A calculated field can be a field that:
The Jira Misc Custom Fields plugin provides a suite of custom fields for use in your custom JIRA workflows.