Keywords are the main concepts of your research topic and/or question.
When you are searching for books and articles on your topic, you will need to select some keywords. Keywords help point your search in the right direction and are like the main ideas in a sentence or paragraph. The words in your research question are perfect starting points.
For example, What are the side effects of chemotherapy? The keywords are side effects and chemotherapy- but there are other words that might help you, such as cancer, radiation, etc.
Terms that mean the same thing (synonyms) are also worthwhile keywords.
Skinny is sometimes referred to as slim or underweight and drug use among athletes is sometimes called doping. Using different terms to locate information will give you greater results.