"Controlled vocabulary" refers to a list of predefined words or phrases used to organize information during the indexing process for retrieval by users during a search. Using a controlled vocabulary promotes consistency and accuracy across a particular database by establishing preferred and variant terms for a given concept. For example, terminology a doctor may use to describe a symptom will often differ from how a patient describes that same symptom. A controlled vocabulary accounts for these variations and suggests the single term or phrase that is the preferred term and will provide the best search results.
There are multiple types of controlled vocabularies, such as thesauri, ontologies, authority files, etc., but the common function across all types of controlled vocabularies is the compilation of synonyms of concepts and linking these concepts together or organizing them into categories.
You can learn more in this Knowledge Base article: