A subject heading is a searchable tag specifically chosen to represent a specific idea or topic that can be said in many ways.
In the English language, there are several different words that communicate the same or similar idea. For instance, the idea of a dog can be expressed as dog, canis lupus familiaris, doggy, pooch, man's best friend, or canine. Same concept, very different words.
This doesn't mean any time an author wants to mention a dog, they have to say canine. Instead, a librarian or even the author of a title will assign subject headings using a list of approved words or phrases, and insert them into the information they give the database. These subject headings then become searchable.
Each database has different headings. This makes sense because a medical database would have a controlled tag for lumbar puncture, but not necessarily for transformational leadership. A business database, however, would be vice-versa.