Keywords are important words/concepts found in your research question or thesis.
A quick and dirty way to pull keywords from a research question/thesis is to choose the most important nouns; all other words are irrelevant.
Using keywords to search will always retrieve more results than phrases or sentences.
Image source: Producer. CC BY-SA 3.0. Wikimedia Commons
You can find keywords:
Image source: Evan-Amos. Public Domain. Wikimedia Commons.
Keywords have a profound impact on search results. Using the right words will speed up the research process, while the wrong ones can bring to it to a painfully screeching halt.
Use the chart above to document keywords related to your topic. Keep it by your side when you start your research.