Keywords are the most important words or phrases that describe your topic. For example, if your paper is based on the question, "Are we taking it too far by blaming fast food restaurants for obesity?" then your keywords could be: fast food and obesity. A search strategy in a database may look like this: "fast food" and obesity and (america or united states)
A Boolean search combines keywords with operators (AND, NOT, and OR) to produce more relevant results when you are searching.