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.
Thinking of synonyms (and searching them OR'd together) will always expand the number of results you retrieve (see Search Strategies).
Image source: Producer. CC BY-SA 3.0. Wikimedia Commons
Before you begin searching for information, you must identify keywords related to your topic. Find keywords:
Image source: Evan-Amos. Public Domain. Wikimedia Commons.