In keyword searching, you have to choose words that reflect the most important concepts in your topic area. At the same time, you must choose words that actually appear in the records for the articles, books and other sources you're looking for.
Sometimes you can predict in advance what the most effective keywords will be. Sometimes, you have to do some browsing to find out how people in the conversation you want to enter talk about your issue. As you will discover in the video below, different resources may use drastically different terms for the same thing: