Subject terms are:
Another method for finding related terms. When conducting your research, look at the records of relevant articles, and see what terms have been assigned. You can use those terms to find similar articles.
Controlled vocabulary. These are used to classify and organize information contained within a database or library catalogue. This is similar to tagging, in which each record in a database is tagged with subject headings selected from a list of approved words and phrases.
A method for limiting your search. Limiting your search to locate terms within a specific field will produce fewer but more relevant and focused results, especially as subject headings reflect the main focus of that particular resource. You can limit your search to the "subject" field to look only for these terms.
See the figure below for an example of subject keywords found in a library database record: