In searching databases, keyword searching is similar to Google searching- you're looking through the entire record (author, title, full text, etc.) to locate instances of the term you use. The more times the term is used, the higher the 'relevance' of the result.
The subject of an article is the 'aboutness-' what is the articles actually talking about. the crucial difference is that subject terms are usually in a special list (thesaurus) while keywords can be anything. Choose a subject term not on the list, zero results.
Bottom line- keywords produce more results with less accuracy, subject terms produce fewer, but relevant, results.
The trick is to find the right subject terms. I suggest starting out using keywords- this lets you know what people are actually writing about, and all articles are cataloged under multiple subject headings. People want you to find their stuff, and they will help you get there. Once you find some subject terms, the field gets narrowed quickly.