Unlike keywords which can be found in any part of an item record (the title, description or article), terms in a controlled vocabulary are assigned to a book, article, or document based on the subject or content of the item. These terms are "controlled" by a pre-defined list of subject headings or descriptors, so that items with the same subject are assigned the same term. When you search terms in the controlled vocabulary, you will get every item that has been assigned that term.
You can find controlled vocabulary terms in the database record for an article. Click on the article title. The terms are generally listed as "Subjects" in the record for that article.
Sometimes, you can find controlled vocabulary before you start searching. Book subjects are contained in the Library of Congress Subject Headings. Most EBSCO databases contain a Thesaurus or Subject or Subject Term list that can be searched or browsed to find the controlled vocabulary used for a particular subject.
Controlled vocabulary for the term "shell shock:"