What are subject headings?
When you use Google or Google Scholar or databases such as Scopus or PubMed, you usually type in the obvious-to-you words to get some results. But does everyone else writing on or researching the same topic use the same words that you do?
For example, you might use teens but someone else might use adolescents.
Subject headings can help.
Subject headings are standard terms for particular themes/topics.
When the details of an article (or book) are entered into a database, it is tagged with subject headings for the main topics that it covers, no matter how the author refers to the topic.
Different databases use different subject headings. So in another database, the Subject Heading may be Teenagers.