subject words = less hits & better
This means a book or article has been described using a set of subject words. Not all words are subject words. Each catalog/database/resource has its own subject words, sometimes called descriptors. The subject words used in the library catalog for your topic will often translate well to other databases.
keywords = more hits & sometimes worse
A keyword means the word appears anywhere in the information--title, author, notes, description, publisher, etc., or in the full text when that is available.
Full text means ebooks, pdfs, etc. available online. It is the opposite of a citation or reference in which the item is not immediately available online and the book, article, document is described only superficially-- title, author, publication details, subject words or descriptors, possibly an abstract or summary. Searching full text (through JSTOR, Google Books, etc) should be your last resort, not for your first step. Generally you get too many hits to evaluate.
Better searching generally means searching citations in databases which will give you better and more manageable number of hits which can be more easily evaluated.