Journal Terms Lists are used by EndNote to automatically convert an abbreviated journal title into a full version or a full journal title into an abbreviated version. They are stored in the term lists folder in the EndNote Program Files.
EndNote: Source of Terms Lists (clarivate.com)
EndNote: Generate full or abbreviated journal names (EndNote 20) (clarivate.com)
Your referencing style will specify if your journal titles need to be in full or abbreviated version. For example Vancouver uses abbreviated journal names whereas APA 6th uses the full journal name.