I have covered a similar topic before: Filter API result by nested field - #8 by Denney-tech
I would treat “fields” as a generic term to refer to the <field_name> that you can filter api queries by with ?. You can usually find what fields are actually filterable by looking under search_fields, related_search_fields, summary_fields or other *_fields.
If you go to the jobs api in your browser, /api/controller/v2/jobs/ (in AAP 2.5 for example), and click on “options”, it will show summary_fields you can filter by and some generic examples. So for example, you could use /api/controller/v2/jobs/?organization_id=1 to get all jobs in the default org. You can filter by nested fields using double underscores such as /api/controller/v2/jobs/?created_by__username=foo_user to find all jobs run by a particular user. Some of the valid fields may happen to be listed in addition to the summary_fields, but not all of them will be valid (such as launched_by).