Keywords are simply the words you put into a search box. Because search engines and databases can't read your mind (yet), it's a good idea to think carefully about the terms you use and how you structure them in a search if you want to retrieve all of the most relevant sources.
It's best to distill your research question down to two or three concepts, each represented by one keyword. Once you have those, think of broader, narrower, and related terms, and use them all in different combinations in your searches to maximize your chances of finding relevant articles.
Sometimes the terms Keywords and Subjects are used interchangeably. That's fine in most cases, but when you search library databases, they mean very different things. Check it out:
Think of keywords as the terms you might use for a Google search. They're terms you think of yourself, sometimes off the top of your head, in hopes that they appear somewhere in the items you're trying to retrieve. Keywords are a great way to start a search.
Subjects, on the other hand, are terms applied to items by people in the know: librarians, bibliographers, indexers, authors, publishers, etc. Subject headings describe what an item is actually about, and they tend to be "controlled," meaning that the same subject heading is used across a given index or database. Once you identify a relevant subject heading, that subject heading will lead you to many other items about the same things.
Start with a good keyword search in a library catalog or database and find one *perfect* (or nearly-perfect) article. If that article has subject headings, descriptors, or keywords assigned (and it should), use those to find other similar items.