Controlled vocabulary legislate the use of predefined, authorised terms that have been preselected by the designer of the vocabulary, in contrast to natural language vocabularies, where there is no restriction on the vocabulary.
Subject headings are a controlled vocabulary. A single term or phrase is determined by an authority (Library Congress or a database vendor) to describe a subject. For example, try searching for the subject "sources" in West-Cat and note that the subject term "sources" is added to a library item when it is considered by the Library of Congress to include primary sources. This can be very useful as you look for history in primary sources.
Library of Congress Subject Headings (LCSH)