Term lists are like authority files - by default term lists for Authors, Journals, and Keywords are already set up when you create a new library. You can access the term lists from Tools - Open Term Lists in the top menu.
As you enter new references into your library - EndNote automatically updates the term lists so that the Authors, Journals, and Keywords lists include all the author names, journal names and keywords entered into your references. When you enter a new term into any of these 3 fields - EndNote attempts to autocomplete it for you. New terms appear in red text in your references and will be added to the term lists when you close or save the reference.
Any term in a term list can be modified or deleted without affecting any of the references in the EN library.
N.B. Editing a term in a term list does not change all occurrences of that term in your library. Use the Find and Replace command from the Edit menu to find the old term in your references and replace it with the new one.
PubMed sends abbreviated journal titles to EndNote. While this is useful for some citation styles, for other styles such as Harvard or APA it doesn't allow the full titles of journals to be displayed in bibliographies. Here is a short video on how to ensure that you can have either full journal titles or abbreviated journal titles in your bibliographies.