A controlled vocabulary refers to a list of standardized terms that can be used to effectively search databases when conducting research; it can also be referred to as subject terms, subject headings, descriptors, or as a thesaurus. These are terms used by cataloguers in a library setting and database indexers to describe each topic; this allows researchers to identify more information resources on a certain topic by utilizing these terms.
The above subject terms come from this article: Community mental health staff’s perspectives on telehealth for veterans with serious mental illness during the COVID-19 pandemic.
Searching in a database can sometimes walk the fine line of art and science. When you use keywords to search a database, you will often be provided more results on account of the keywords targeting all the fields, such as the journal title, the title of the journal article, or abstract. A drawback to this approach is the increased number of irrelevant results you will encounter in your search and the added possibility of missing out on potentially important resources because the correct term wasn't used in the search.
The controlled vocabulary approach will narrow the number of results in a database search but will be much more relevant and focused. A downside to using controlled vocabulary is newer jargon, slang, or terminology could be excluded from the results.