Think of subject headings as labels or tags that someone has used to identify the subject of a book or article. The subject headings are standardized so that only one term is used for a specific subject. They work similarly to #hashtags in Twitter, subject headings categorize and pull together all articles on a topic.
The Medical Center Library book and ebook collections follow the MeSH (Medical Subject Headings) determined by the U.S. National Library of Medicine, while Pius XII follows the Library of Congress Subject Headings as standards. Some databases, such as PubMed, will follow established subject headings (PubMed uses MeSH) while other databases, such as CINAHL, follow their own proprietary subject headings.
Results will be fewer, but most will be on point—only searches the subject heading field, NOT the title/abstract fields or any other fields.