Keyword search in library database (like Multisearch or JSTOR) finds you words in title, abstract, or body text of article, book, etc.
Subject search in library database finds you more of the same content.
A human indexer assigns subject terms to each article, book, and video, using a controlled vocabulary. It's like a strict hashtag system designed to help researchers and students find content faster.
Expert users start with keyword searches, then switch to searching interesting subject terms.