Subject headings (or subject terms, index terms, etc.) are a controlled vocabulary used by databases in order to describe concepts and things. In modern terms, they're like hashtags.
They are the organizational framework of databases (at least those databases that use them), and they can be used to precisely and systematically search those databases.
The two videos to the right cover the use of subject headings on PubMed. While the videos are specific to PubMed, they should capture the essence of subject heading searching and could be applicable to other databases that use subject headings (CINAHL Plus with Full Text, APA PsycINFO, ERIC, etc.).