Library database resources are indexed by subject headings
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.
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.
Subject headings may also have hierarchical or categorized concepts. For example, a MeSH search for Craniofacial Abnormalities will also find its subheading, cleft palate. However, a MeSH search for cleft palate will not find other craniofacial abnormalities.
Subject Heading searches pull all records with a specific concept to the result list. Results may be fewer, but may also find articles missed in keyword searches.
Subject searching only searches the subject heading field, NOT the title/abstract fields or any other fields.