Subject terms are controlled (preferred) vocabulary similar to tagging, they are assigned by professionals. Searching using subject terms help you find all of the documents about a topic even if the document doesn't contain the words you're searching for.* For example, if you were looking for articles about "cats" using just keywords, you might not find an article that only used the word "feline". Searching with subject terms retrieves any article about that subject regardless of the specific words authors use.
*Because of variations in indexing (assigning subject headings), your search may still miss some articles on your topic.
Using subject terms usually improves the relevance of search results.