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.
How do you decide which search results are relevant to your research?
Good searching is not a matter of typing a word or two in a search box and getting 100,000 hits with one click. When you're searching, you're actually learning about a topic--how much has been written about it and in what ways. A good search result is generally between 20-100 relevant items.
When you get too many results:
When you don't get enough results:
When you're not getting the right kind of results:
Don't forget to look at footnotes and bibliographies of relevant articles and books, including material that was assigned in class.
Consult with your professor. They will think more highly of you for seeking their advice.
Get advice from Ask a Librarian.