Permanent names are a property of Tracker Fields info that was introduced to improve API stability for code and webservices interacting with Trackers. They reduce the amount of Tracker Field IDs you will have to handle while configuring Tiki as well.
The permanent names are generated automatically from the field name on creation, provided that you have the Basic Search preference disabled. They can be modified when editing the field in the Advanced section.
In Tiki24, there are also permanent names for trackers (not just fields)