Subject headings are "controlled vocabulary". A controlled vocabulary makes a database easier to search by placing all variations of an idea under a single word or phrase, eliminating guess work and making searches more efficient. For example, if I want to search for English Setters I could use: dogs, canines, hunting dogs, English dog breeds, etc. The controlled vocabulary for this concept is Dogs, which provides an organized list of available materials:
If I really only want books about English Setters then searching in Keyword for "english setters" might be a better choice. Keywords come from several parts of a record, not just the subject field; in the example below english setters was in the Note field.